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Antimicrobial susceptibility testing (AST) and associated clinical outcomes in individuals with cystic fibrosis: A systematic review

Open ArchivePublished:January 29, 2019DOI:https://doi.org/10.1016/j.jcf.2019.01.008

      Highlights

      • Twenty studies compared bacterial isolate susceptibilities and treatment outcomes.
      • Three studies showed evidence of a relationship between in vitro testing and response.
      • Seventeen studies showed no relationship between susceptibility testing and response.
      • Antimicrobial susceptibility test results do not predict CF treatment responses.

      Abstract

      Background

      Antimicrobial susceptibility testing (AST) is a cornerstone of infection management. Cystic fibrosis (CF) treatment guidelines recommend AST to select antimicrobial treatments for CF airway infection but its utility in this setting has never been objectively demonstrated.

      Methods

      We conducted a systematic review of primary published articles designed to address two PICO (patient, intervention, comparator, outcome) questions: 1) “For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection predictable from AST results available at treatment initiation?” and 2) “For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection affected by the method used to guide antimicrobial selection?” Relationships between AST results and clinical response (changes in pulmonary function, weight, signs and symptoms of respiratory tract infection, and time to next event) were assessed for each article and results were compared across articles when possible.

      Results

      Twenty-five articles describing the results of 20 separate studies, most of which described Pseudomonas aeruginosa treatment, were identified. Thirteen studies described pulmonary exacerbation (PEx) treatment and seven described ‘maintenance’ of chronic bacterial airways infection. In only three of 16 studies addressing PICO question #1 was there a suggestion that baseline bacterial isolate antimicrobial susceptibility was associated with clinical response to treatment. None of the four studies addressing PICO question #2 suggested that antimicrobial selection methods influenced clinical outcomes.

      Conclusions

      There is little evidence that AST predicts the clinical outcome of CF antimicrobial treatment, suggesting a need for careful consideration of current AST use by the CF community.

      1. Introduction

      Antimicrobial susceptibility testing (AST) is an important component of infection management intended to guide the selection of antimicrobial therapies. AST is most predictive of clinical response among “immunocompetent patients with monomicrobic bacterial infections treated with a single antimicrobial agent which is administered parenterally in circumstances in which the penetration of drug to the site of infection is predictable.” [
      • Doern G.
      • Brecher S.M.
      The clinical predictive value (or lack thereof) of the results of in vitro antimicrobial susceptibility tests.
      ] In these situations, ~90% of infections with bacteria identified as ‘susceptible’ by AST respond to antimicrobial treatment, versus ~60% of infections with ‘resistant’ bacteria [
      • Rex J.H.
      • Pfaller M.A.
      Has antifungal susceptibility testing come of age?.
      ].
      Antimicrobial treatment for bacterial infection is a cornerstone of cystic fibrosis (CF) management, whether it be antibacterial prophylaxis [
      • Smyth A.R.
      • Rosenfeld M.
      Prophylactic anti-staphylococcal antibiotics for cystic fibrosis.
      ], ‘eradication’ (conversion to culture negativity) of early respiratory tract infections [
      • Langton Hewer S.C.
      • Smyth A.R.
      Antibiotic strategies for eradicating Pseudomonas aeruginosa in people with cystic fibrosis.
      ], treatment of acute pulmonary exacerbations (PEx) [
      • Flume P.A.
      • VanDevanter D.R.
      Exacerbations.
      ], or ‘maintenance treatment’ of chronic bacterial respiratory tract infections [
      • Flume P.A.
      • O'Sullivan B.P.
      • Robinson K.A.
      • Goss C.H.
      • Mogayzel Jr., P.J.
      • Willey-Courand D.B.
      • et al.
      Cystic fibrosis pulmonary guidelines. Chronic medications for maintenance of lung health.
      ]. CF treatment guidelines support the use of AST to guide antimicrobial treatment [
      Treatment of pulmonary exacerbation of cystic fibrosis.
      ,], although it has been suggested that AST may not be predictive of CF clinical response [
      • Döring G.
      • Flume P.
      • Heijerman H.
      • Elborn J.S.
      • Consensus Study Group
      Treatment of lung infection in patients with cystic fibrosis: current and future strategies.
      ].
      If AST is not predictive of antimicrobial treatment response in CF, it may be in the community's interest to be more selective in the use of this resource-intensive test. As part of the aims of the Antimicrobial Resistance in Cystic Fibrosis International Working Group, we have conducted a systematic review of the primary literature to address two PICO (patient, intervention, comparator, outcome) research questions:
      #1. For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection predictable from AST results available at treatment initiation?
      #2. For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection affected by the method used to guide antimicrobial selection?

      2. Methods

      A systematic search of the PubMed database was performed according to Cochrane guidelines [
      ]. Peer-reviewed, primary research articles published in English describing clinical outcomes after antimicrobial treatment of individuals with CF were identified using the search strategy: (cystic fibrosis terms) AND (susceptibility, resistance, sensitivity terms) AND (antimicrobial, antibiotic, anti-bacterial terms) (see online supplement). In a primary screen, a single researcher (JK) excluded articles from search results that were non-English language, did not report CF treatment outcomes, lacked an abstract, or were reviews, meta-analyses, or case studies. Next, article abstracts with or without entire manuscripts were reviewed by four researchers independently (teams of four drawn from DV, RS, MP, MT, AS, SM, JK). Unanimous team agreement was required for article inclusion. Uncertainties/discrepancies were reconciled in a tertiary screen by discussion of full texts among research teams.
      Included articles reported study results in which 1) antimicrobial treatments were administered and clinical outcomes were compared among groups based on AST results available at treatment initiation and/or 2) differing methodologies were used to guide antimicrobial selection for treatment of study groups and clinical outcomes were compared between treated groups. Data from multiple articles describing outcomes from the same study were merged for analyses. Treatment response differences among study subject groups were assessed using investigators' statistical tests (when available) and their associated conclusions, with only clinical responses (e.g., spirometry, signs and symptoms of respiratory tract infection, days in hospital, and/or time to next event) studied. Study quality was assessed by two reviewers (RS and DV) using the GRADE criteria [
      ,
      • GRADEpro G.D.T.
      GRADEpro guideline development tool [Software].
      ] for PICO questions by outcome and study design (see online supplement).

      3. Results

      3.1 Search results

      An initial PubMed search yielded 1214 citations, of which 441 were excluded by primary screen. Twenty-five articles [
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Goldmann D.A.
      Randomized, double-blind evaluation of azlocillin for the treatment of pulmonary exacerbations of cystic fibrosis.
      ,
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Taneja A.
      • Murphy P.
      • et al.
      Clinical and bacteriological responses to three antibiotic regimens for acute exacerbations of cystic fibrosis: ticarcillin-tobramycin, azlocillin-tobramycin, and azlocillin-placebo.
      ,
      • Schaad U.B.
      • Desgrandchamps D.
      • Kraemer R.
      Antimicrobial therapy of Pseudomonas pulmonary exacerbations in cystic fibrosis. A prospective evaluation of netilmicin plus azlocillin versus netilmicin plus ticarcillin.
      ,
      • Bosso J.A.
      • Black P.G.
      Controlled trial of aztreonam vs. tobramycin and azlocillin for acute pulmonary exacerbations of cystic fibrosis.
      ,
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ,
      • Master V.
      • Roberts G.W.
      • Coulthard K.P.
      • Baghurst P.A.
      • Martin A.
      • Roberts M.E.
      • et al.
      Efficacy of once-daily tobramycin monotherapy for acute pulmonary exacerbations of cystic fibrosis: a preliminary study.
      ,
      • Smith A.L.
      • Fiel S.B.
      • Mayer-Hamblett N.
      • Ramsey B.
      • Burns J.L.
      Susceptibility testing of Pseudomonas aeruginosa isolates and clinical response to parenteral antibiotic administration: lack of association in cystic fibrosis.
      ,
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ,
      • Foweraker J.E.
      • Laughton C.R.
      • Brown D.F.
      • Bilton D.
      Comparison of methods to test antibiotic combinations against heterogeneous populations of multiresistant Pseudomonas aeruginosa from patients with acute infective exacerbations in cystic fibrosis.
      ,
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ,
      • Hurley M.N.
      • Ariff A.H.
      • Bertenshaw C.
      • Bhatt J.
      • Smyth A.R.
      Results of antibiotic susceptibility testing do not influence clinical outcome in children with cystic fibrosis.
      ,
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ,
      • Lam J.C.
      • Somayaji R.
      • Surette M.G.
      • Rabin H.R.
      • Parkins M.D.
      Reduction in Pseudomonas aeruginosa sputum density during a cystic fibrosis pulmonary exacerbation does not predict clinical response.
      ,
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ,
      • Ramsey B.W.
      • Pepe M.S.
      • Quan J.M.
      • Otto K.L.
      • Montgomery A.B.
      • Williams-Warren J.
      • et al.
      Intermittent administration of inhaled tobramycin in patients with cystic fibrosis.
      ,
      • Burns J.L.
      • Van Dalfsen J.M.
      • Shawar R.M.
      • Otto K.L.
      • Garber R.L.
      • Quan J.M.
      • et al.
      Effect of chronic intermittent administration of inhaled tobramycin on respiratory microbial flora in patients with cystic fibrosis.
      ,
      • Hodson M.E.
      • Gallagher C.G.
      • Govan J.R.
      A randomised clinical trial of nebulised tobramycin or colistin in cystic fibrosis.
      ,
      • Govan J.R.
      Insights into cystic fibrosis microbiology from the European tobramycin trial in cystic fibrosis.
      ,
      • Moss R.B.
      Administration of aerosolized antibiotics in cystic fibrosis patients.
      ,
      • LiPuma J.J.
      Microbiological and immunologic considerations with aerosolized drug delivery.
      ,
      • Moss R.B.
      Long-term benefits of inhaled tobramycin in adolescent patients with cystic fibrosis.
      ,
      • Etherington C.
      • Hall M.
      • Conway S.
      • Peckham D.
      • Denton M.
      Clinical impact of reducing routine susceptibility testing in chronic Pseudomonas aeruginosa infections in cystic fibrosis.
      ,
      • Moskowitz S.M.
      • Emerson J.C.
      • McNamara S.
      • Shell R.D.
      • Orenstein D.M.
      • Rosenbluth D.
      • et al.
      Randomized trial of biofilm testing to select antibiotics for cystic fibrosis airway infection.
      ,
      • Geller D.E.
      • Flume P.A.
      • Staab D.
      • Fischer R.
      • Loutit J.S.
      • Conrad D.J.
      Levofloxacin inhalation solution (MP-376) in patients with cystic fibrosis with Pseudomonas aeruginosa.
      ,
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ] describing 20 separate studies met inclusion criteria from the remaining 773 articles (Fig. 1). Sixteen studies (21 articles) addressed PICO question #1 and four studies addressed PICO question #2. No studies examined relationships between AST and treatment of first bacterial infection, 13 examined antimicrobial treatment of PEx (Table 1), and seven examined maintenance antimicrobial treatment of chronic airway infection (Table 2).
      Fig. 1
      Fig. 1Disposition of citations from the initial PubMed search yield.
      Table 1PEx antimicrobial treatment: relationships between AST results and clinical response.
      Authors, yearCountryDesignAntimicrobials studied; AST-related conclusionsPICO questionReference
      McLaughlin et al., [
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Goldmann D.A.
      Randomized, double-blind evaluation of azlocillin for the treatment of pulmonary exacerbations of cystic fibrosis.
      ,
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Taneja A.
      • Murphy P.
      • et al.
      Clinical and bacteriological responses to three antibiotic regimens for acute exacerbations of cystic fibrosis: ticarcillin-tobramycin, azlocillin-tobramycin, and azlocillin-placebo.
      ]
      USARandomized, blindedIV azlocillin/tobramycin, ticarcillin/tobramycin, and azlocillin; “Patients with sputum bacteria that were resistant to one or both of the antibiotics… fared no worse in terms of pulmonary function”113,14
      Schaad et al., [
      • Schaad U.B.
      • Desgrandchamps D.
      • Kraemer R.
      Antimicrobial therapy of Pseudomonas pulmonary exacerbations in cystic fibrosis. A prospective evaluation of netilmicin plus azlocillin versus netilmicin plus ticarcillin.
      ]
      SwitzerlandRandomizedIV netilmicin/azlocillin, netilmicin/ticarcillin; “No significant correlation between severity of CF, antibiotic susceptibility of the initial Pseudomonas isolates, or measured netilmicin concentrations and… clinical improvements”.115
      Bosso et al., [
      • Bosso J.A.
      • Black P.G.
      Controlled trial of aztreonam vs. tobramycin and azlocillin for acute pulmonary exacerbations of cystic fibrosis.
      ]
      USARandomizedIV aztreonam, tobramycin, azlocillin. “Patients with antibiotic-resistant Pa or P. cepacia… responded as well to treatment as those with only susceptible isolates.”116
      Wolter et al., [
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ]
      AustraliaRetrospectiveIV ceftazidime/tobramycin; No statistically significant differences in ppFEV1 and FVC responses between patients with tobramycin- or ceftazidime-resistant versus susceptible Pa isolates117
      Master et al., [
      • Master V.
      • Roberts G.W.
      • Coulthard K.P.
      • Baghurst P.A.
      • Martin A.
      • Roberts M.E.
      • et al.
      Efficacy of once-daily tobramycin monotherapy for acute pulmonary exacerbations of cystic fibrosis: a preliminary study.
      ]
      AustraliaRandomized, blindedIV tobramycin, tobramycin/ceftazidime; The proportion of events with PFT response to IV tobramycin treatment was 84.7% although >80% of baseline Pa isolates had tobramycin MICs above the tobramycin parenteral breakpoint.118
      Smith et al., [
      • Smith A.L.
      • Fiel S.B.
      • Mayer-Hamblett N.
      • Ramsey B.
      • Burns J.L.
      Susceptibility testing of Pseudomonas aeruginosa isolates and clinical response to parenteral antibiotic administration: lack of association in cystic fibrosis.
      ]
      USARetrospectiveIV tobramycin/ceftazidime; In vitro tobramycin and/or ceftazidime resistance did not significantly impact ppFEV1 response to treatment119
      Aaron et al., [
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ]
      Canada/AustraliaRandomized, blindedTwo IV antimicrobials ± inhaled tobramycin; Time to next PEx and changes in ppFEV1, FVC, and dyspnea scores did not differ between subjects treated with antimicrobials chosen by physicians versus those chosen by multiple combination bactericidal testing (MCBT).220
      Foweraker et al., [
      • Foweraker J.E.
      • Laughton C.R.
      • Brown D.F.
      • Bilton D.
      Comparison of methods to test antibiotic combinations against heterogeneous populations of multiresistant Pseudomonas aeruginosa from patients with acute infective exacerbations in cystic fibrosis.
      ]
      UKRetrospectiveTwo IV antipseudomonal antimicrobials. In vitro susceptibilities and antimicrobial synergy testing did not predict ppFEV1 response to combination antimicrobial treatments.121
      Hubert et al., [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ]
      FranceRandomized, crossoverContinuous infusion versus 3× daily IV ceftazidime with 3× daily tobramycin. Mean ppFEV1 response was not different between subjects with ceftazidime-susceptible versus with -resistant Pa isolates treated by continuous infusion but was poorer among patients with resistant isolates treated with ceftazidime intermittently.122
      Hurley et al., [
      • Hurley M.N.
      • Ariff A.H.
      • Bertenshaw C.
      • Bhatt J.
      • Smyth A.R.
      Results of antibiotic susceptibility testing do not influence clinical outcome in children with cystic fibrosis.
      ]
      UKRetrospectiveMultiple IV antimicrobials; No association was observed between ppFEV1 or BMI change or time to next PEx and concordance between proportions of antimicrobials to which bacterial isolates were resistant (none, some, or all).123
      Parkins et al., [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ]
      UKRetrospectiveTwo IV antipseudomonal antimicrobials (a beta-lactam and an aminoglycoside or polymyxin); Significant univariate relationships observed between rate of investigator-defined treatment failure and numbers of antimicrobial agents administered with isolate resistance were not observed in multivariate modeling correcting for severity of initial presentation and inflammatory status.124
      Lam et al., [
      • Lam J.C.
      • Somayaji R.
      • Surette M.G.
      • Rabin H.R.
      • Parkins M.D.
      Reduction in Pseudomonas aeruginosa sputum density during a cystic fibrosis pulmonary exacerbation does not predict clinical response.
      ]
      CanadaRetrospectiveMultiple antimicrobials; PEx treatment failure was not influenced by the number of antibiotics in combination treatments to which Pa isolates were susceptible.125
      Yau et al., [
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ]
      CanadaRandomized, blindedTwo IV antimicrobials with different mechanisms of action chosen based on susceptibility test results. No difference in ppFEV1 change was observed between subjects treated with antimicrobials chosen based on biofilm versus planktonic growth susceptibility tests.226
      BMI–body mass index; IV– intravenous; MIC – minimum inhibitory concentration; PaP. aeruginosa; ppFEV1 – percent predicted forced expiratory volume in 1 s
      Table 2Antimicrobial maintenance of chronic respiratory tract infections: relationships between AST results and clinical response.
      Authors, YearCountryDesignAntimicrobials studied; AST-related conclusionsPICO QuestionReference
      Ramsey et al., [
      • Ramsey B.W.
      • Pepe M.S.
      • Quan J.M.
      • Otto K.L.
      • Montgomery A.B.
      • Williams-Warren J.
      • et al.
      Intermittent administration of inhaled tobramycin in patients with cystic fibrosis.
      ]

      Burns et al. [
      • Burns J.L.
      • Van Dalfsen J.M.
      • Shawar R.M.
      • Otto K.L.
      • Garber R.L.
      • Quan J.M.
      • et al.
      Effect of chronic intermittent administration of inhaled tobramycin on respiratory microbial flora in patients with cystic fibrosis.
      ]
      USARandomized, blindedInhaled tobramycin; Mean ppFEV1 improvement was numerically, but not significantly greater among subjects with tobramycin-susceptible Pa isolates at baseline than those with tobramycin-resistant isolates.127,28
      Hodson et al., [
      • Hodson M.E.
      • Gallagher C.G.
      • Govan J.R.
      A randomised clinical trial of nebulised tobramycin or colistin in cystic fibrosis.
      ]

      Govan et al., [
      • Govan J.R.
      Insights into cystic fibrosis microbiology from the European tobramycin trial in cystic fibrosis.
      ]
      UK, IrelandRandomizedInhaled tobramycin and colistimethate; Clinical response from baseline after 28 days of treatment was not predicted by baseline Pa isolate susceptibilities.129, 30
      Moss [
      • Moss R.B.
      Administration of aerosolized antibiotics in cystic fibrosis patients.
      ]

      LiPuma, [
      • LiPuma J.J.
      Microbiological and immunologic considerations with aerosolized drug delivery.
      ]

      Moss [
      • Moss R.B.
      Long-term benefits of inhaled tobramycin in adolescent patients with cystic fibrosis.
      ]
      USAOpen-labelInhaled tobramycin; Mean ppFEV1 response to inhaled tobramycin treatment was not predicted by Pa isolate tobramycin susceptibility after 92 weeks of treatment. Proportions of patients with ppFEV1 increased from baseline were similar across isolate tobramycin susceptibility groups (<16 μg/ml, 16–64 μg/mL, >64 μg/mL).131, 32, 33
      Etherington et al., [
      • Etherington C.
      • Hall M.
      • Conway S.
      • Peckham D.
      • Denton M.
      Clinical impact of reducing routine susceptibility testing in chronic Pseudomonas aeruginosa infections in cystic fibrosis.
      ]
      UKRetrospectiveMultiple IV antimicrobials; Less frequent susceptibility testing had no effect on clinical responses to scheduled IV antimicrobial treatments for Pa suppression.234
      Moskowitz et al., [
      • Moskowitz S.M.
      • Emerson J.C.
      • McNamara S.
      • Shell R.D.
      • Orenstein D.M.
      • Rosenbluth D.
      • et al.
      Randomized trial of biofilm testing to select antibiotics for cystic fibrosis airway infection.
      ]
      USARandomizedMultiple IV antimicrobials; No differences in mean change in ppFEV1 from baseline were observed between subjects with antipseudomonal antimicrobials selected by biofilm susceptibility testing versus traditional broth testing.235
      Geller et al., [
      • Geller D.E.
      • Flume P.A.
      • Staab D.
      • Fischer R.
      • Loutit J.S.
      • Conrad D.J.
      Levofloxacin inhalation solution (MP-376) in patients with cystic fibrosis with Pseudomonas aeruginosa.
      ]
      North America and EuropeRandomized, blindedInhaled levofloxacin; No association was observed between baseline levofloxacin MIC and ppFEV1 response following 28 days of inhaled levofloxacin treatment.136
      Mazurek et al., [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]
      EuropeRandomized, open-labelInhaled tobramycin; Subjects with only tobramycin-susceptible Pa isolates (MIC <8 μg/mL) had statistically greater mean ppFEV1 responses than those with at least one tobramycin-resistant isolate at some study visits over one year. Subjects with at least one tobramycin-resistant Pa isolate, however, had a positive mean ppFEV1 change from baseline at 10/11 study visits over one year.137
      IV – intravenous; MIC – minimum inhibitory concentration; PaP. aeruginosa; ppFEV1 – percent predicted forced expiratory volume in 1 s.
      Descriptions of treatment-associated clinical benefits varied across studies. Several studies identified a categorical variable of response to treatment, including some variant of treatment-associated change in percent predicted forced expiratory volume in 1 s (ppFEV1), either as absolute or relative change from treatment initiation or as a follow-up value relative to a pre-treatment value. Antimicrobial susceptibility reporting also varied widely across studies, ranging from reporting of single bacterial isolates per patient as resistant/susceptible to minimum inhibitory concentrations (MICs) of antimicrobials for multiple isolates per patient.
      Included studies were of moderate or low quality by GRADE criteria, with downgrading primarily a result of either indirectness of the study design to address the question or imprecision in assessing outcomes. There was no downgrading due to risk of bias, as most randomized trials were double-blinded with adequate accounting for subjects and observational studies included specific eligibility criteria with no serious identified flaws in measurement (see online supplement).

      3.1.1 Treatment of PEx

      Fourteen articles described seven prospective and six retrospective PEx treatment studies. Eleven studies addressed PICO question #1 and two addressed PICO question #2 (Fig. 1; Table 1).
      PICO Question #1: Is clinical response to antimicrobial treatment of bacterial airways infection predictable from AST results available at treatment initiation?
      Two studies suggested AST and PEx treatment response correlations, while 11 did not. The first study that suggested correlation was a prospective crossover study of continuous versus intermittent intravenous (IV) ceftazidime infusion (coupled with thrice-daily IV tobramycin) [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ]. Subjects with ceftazidime-susceptible Pseudomonas aeruginosa (Pa) isolates (N = 28) receiving thrice-daily IV ceftazidime for PEx experienced a mean 8.1 (8.4) ppFEV1 improvement after treatment versus only a 1.7 (5.6) ppFEV1 improvement among subjects with ceftazidime-resistant isolates (N = 15; Fig. 2, left panel). In contrast, when the same population was treated for PEx with continuously-infused ceftazidime, subjects with ceftazidime-susceptible Pa isolates (N = 26) had a mean 7.9 (SD = 9.7) ppFEV1 improvement and those with resistant isolates (N = 18) had a 6.2 (6.6) ppFEV1 improvement [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ]. The difference in mean ppFEV1 responses between continuous and thrice daily ceftazidime infusions among subjects with ceftazidime-resistant isolates was statistically significant (P < 0.05).
      Fig. 2
      Fig. 2Mean ppFEV1 changes from combination IV antimicrobial treatment initiation for PEx stratified by in vitro bacterial susceptibility test results. Left: Responses to combined IV ceftazidime/ tobramycin treatments stratified by whether ceftazidime was administered three times daily in short infusions (left) or continuously infused and (right) and by Pa isolate ceftazidime susceptibilities (susceptible: MICs ≤4 μg/mL, intermediate: MICs 8–32 μg/mL, resistant: MICs >32 μg/mL) [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ]. Center: Responses to any IV treatments and IV tobramycin/ceftazidime treatments by susceptibilities of most resistant Pa isolates to each drug (isolates susceptible to all, some, or none of administered antimicrobials) [
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ]. Left: Responses to combined IV antimicrobial treatments by Pa isolate susceptibilities (isolates susceptible to all, some, or none of administered antimicrobials) [
      • Hurley M.N.
      • Ariff A.H.
      • Bertenshaw C.
      • Bhatt J.
      • Smyth A.R.
      Results of antibiotic susceptibility testing do not influence clinical outcome in children with cystic fibrosis.
      ]. Error bars are 95% confidence intervals (CI); numbers in parentheses indicate sample sizes; MIC, minimum inhibitory concentration; Pa, Pseudomonas aeruginosa; ppFEV1, percent predicted forced expiratory volume in 1 s; R/S, resistant/susceptible; TID, three times daily.
      A second study suggesting an AST/response correlation analyzed PEx treatment failures defined as antibiotic regimen change, treatment >20 days, a recurrent event within <45 days, or failure to recover >90% of baseline FEV1 [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ]. Studying 452 PEx treatments among 101 patients, investigators reported a significant (P = 0.018) inverse correlation between treatment failure rates and the number of antimicrobials used in PEx treatments to which patient bacterial isolates were susceptible [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ]. Twenty-eight of 65 PEx treatments (43%) for which Pa isolates were not susceptible to any administered IV antimicrobials were failures, as were 27% (38/140) of treatments in which isolates were susceptible to one administered IV antimicrobial and 24% (59/245) of treatments where isolates were susceptible to two administered IV antimicrobials. No treatment failures were identified among three PEx treatments for which isolates were susceptible to three administered IV antimicrobials [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ]. Multivariate logistic regression modeling of event-specific risk factors for PEx failure, however, found that PEx “severity” at initial presentation (as relative ppFEV1 decline observed at treatment initiation; P = 0.05) and systemic inflammatory markers (C-reactive protein [CRP] concentration at admission, P = 0.01; white blood cell count at discharge, P = 0.001; CRP concentration at discharge, P = 0.09) were “the most important factors influencing PEx outcomes.” [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ] The presence of multi-drug resistant or pan-resistant Pa isolates (i.e., AST results available at treatment initiation) were not significant factors for PEx treatment failure once these other factors were accounted for.
      Six retrospective studies suggested no significant correlations between Pa isolate antimicrobial susceptibilities and clinical response to PEx treatments. In an analysis of 31 combination IV antimicrobial PEx treatments among 17 patients that included AST of 177 Pa isolates, there was no correlation between isolate susceptibilities and categorical treatment response (as any ppFEV1 improvement from admission) [
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ]. Nevertheless, 12 events in which all Pa isolates were susceptible had a mean 8.3 (7.7) ppFEV1 change compared with a mean 3.3 (4.6) ppFEV1 change among three events where patients had isolates resistant to both administered antimicrobials (Fig. 2, middle panel). Mean ppFEV1 responses were numerically, but not significantly, smaller among PEx events treated with IV tobramycin/ceftazidime when patients had isolates resistant to either antimicrobial than compared to when isolates were susceptible to both antimicrobials (Fig. 2, middle panel) [
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ].
      A second retrospective analysis of PEx treatment failures defined as recovery of <90% of baseline FEV1 reported that the number of antimicrobials used with ‘predicted activity’ (i.e., the number of antibiotics used for which Pa isolates were susceptible by AST) did not affect outcomes among 144 PEx treatments in 46 patients [
      • Lam J.C.
      • Somayaji R.
      • Surette M.G.
      • Rabin H.R.
      • Parkins M.D.
      Reduction in Pseudomonas aeruginosa sputum density during a cystic fibrosis pulmonary exacerbation does not predict clinical response.
      ]. In a retrospective analysis of 77 PEx occurring among subjects receiving inhaled placebo during inhaled tobramycin studies, no correlations were observed between Pa ceftazidime or tobramycin susceptibilities and the proportion of patients observed to have a ≥ 5 ppFEV1 response to IV tobramycin/ceftazidime treatment [
      • Smith A.L.
      • Fiel S.B.
      • Mayer-Hamblett N.
      • Ramsey B.
      • Burns J.L.
      Susceptibility testing of Pseudomonas aeruginosa isolates and clinical response to parenteral antibiotic administration: lack of association in cystic fibrosis.
      ]. An exhaustive analysis of the antimicrobial susceptibility patterns (including use of several synergy test methods) of 128 Pa isolates from nine patients receiving combination IV antimicrobial treatment for PEx concluded that “synergy testing methods appeared to be poor at predicting the clinical… response” of treated patients [
      • Foweraker J.E.
      • Laughton C.R.
      • Brown D.F.
      • Bilton D.
      Comparison of methods to test antibiotic combinations against heterogeneous populations of multiresistant Pseudomonas aeruginosa from patients with acute infective exacerbations in cystic fibrosis.
      ]. An analysis of IV antimicrobial treatment responses as a function of Pa isolate susceptibilities for 103 PEx in 52 patients reported “no association between change in ppFEV1 (P=0.54), change in BMI (P=0.12) or time to next exacerbation (P=0.66) and concordance between antibiotic susceptibility and the antibiotics administered” (Fig. 2, right panel) [
      • Hurley M.N.
      • Ariff A.H.
      • Bertenshaw C.
      • Bhatt J.
      • Smyth A.R.
      Results of antibiotic susceptibility testing do not influence clinical outcome in children with cystic fibrosis.
      ].
      A prospective study of IV once-daily high-dose tobramycin and thrice daily IV tobramycin/ceftazidime assessed 98 treatments among 44 enrolled subjects [
      • Master V.
      • Roberts G.W.
      • Coulthard K.P.
      • Baghurst P.A.
      • Martin A.
      • Roberts M.E.
      • et al.
      Efficacy of once-daily tobramycin monotherapy for acute pulmonary exacerbations of cystic fibrosis: a preliminary study.
      ]. Although investigators did not explicitly comment on a possible discordance between Pa tobramycin susceptibilities and treatment responses, 83 of 98 treatments (84.7%) resulted in pulmonary function improvements despite 43 of 52 Pa isolates (82.7%) being tobramycin-resistant (MIC ≥8 μg/mL) at study entry [
      • Master V.
      • Roberts G.W.
      • Coulthard K.P.
      • Baghurst P.A.
      • Martin A.
      • Roberts M.E.
      • et al.
      Efficacy of once-daily tobramycin monotherapy for acute pulmonary exacerbations of cystic fibrosis: a preliminary study.
      ]. Three additional prospective studies of IV antimicrobial PEx treatments lacked systematic reporting of clinical responses stratified by subject isolate antimicrobial susceptibilities, but noted in discussions that clinical responses did not correlate with antimicrobial susceptibility [
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Goldmann D.A.
      Randomized, double-blind evaluation of azlocillin for the treatment of pulmonary exacerbations of cystic fibrosis.
      ,
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Taneja A.
      • Murphy P.
      • et al.
      Clinical and bacteriological responses to three antibiotic regimens for acute exacerbations of cystic fibrosis: ticarcillin-tobramycin, azlocillin-tobramycin, and azlocillin-placebo.
      ,
      • Schaad U.B.
      • Desgrandchamps D.
      • Kraemer R.
      Antimicrobial therapy of Pseudomonas pulmonary exacerbations in cystic fibrosis. A prospective evaluation of netilmicin plus azlocillin versus netilmicin plus ticarcillin.
      ,
      • Bosso J.A.
      • Black P.G.
      Controlled trial of aztreonam vs. tobramycin and azlocillin for acute pulmonary exacerbations of cystic fibrosis.
      ]. One study noted that “Patients with sputum bacteria that were resistant to one or both of the antibiotics they were receiving fared no worse in terms of pulmonary function than patients with antibiotic-susceptible sputum bacteria” [
      • McLaughlin F.J.
      • Matthews Jr., W.J.
      • Strieder D.J.
      • Sullivan B.
      • Taneja A.
      • Murphy P.
      • et al.
      Clinical and bacteriological responses to three antibiotic regimens for acute exacerbations of cystic fibrosis: ticarcillin-tobramycin, azlocillin-tobramycin, and azlocillin-placebo.
      ]. A second noted that “There was no significant correlation between severity of CF, antibiotic susceptibility of the initial Pseudomonas isolates, or measured netilmicin (an aminoglycoside antibiotic) concentrations and any of these improvements” [
      • Schaad U.B.
      • Desgrandchamps D.
      • Kraemer R.
      Antimicrobial therapy of Pseudomonas pulmonary exacerbations in cystic fibrosis. A prospective evaluation of netilmicin plus azlocillin versus netilmicin plus ticarcillin.
      ]. A third noted that “The presence of antibiotic-resistance… was not inversely related to response” and “Patients with antibiotic-resistant P. aeruginosa or P. cepacia… responded as well to treatment as those with only susceptible isolates.” [
      • Bosso J.A.
      • Black P.G.
      Controlled trial of aztreonam vs. tobramycin and azlocillin for acute pulmonary exacerbations of cystic fibrosis.
      ]
      PICO Question #2: Is clinical response to antimicrobial treatment of bacterial airways infection affected by the method used to guide antimicrobial selection?
      Two prospective randomized, blinded, multicenter studies analyzed the effects of different antimicrobial selection methods for PEx treatment on clinical outcomes [
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ,
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ]. Neither study demonstrated a significant difference in clinical outcomes between AST methods used to guide antimicrobial selection.
      A multicenter Canadian/Australian study of 251 subjects compared mean lung function responses and median time to next PEx among subjects treated with antimicrobials selected by treating physicians with responses for subjects whose antimicrobials were selected based on multiple combination bactericidal antibiotic testing (MCBT) [
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ]. Unlike traditional AST that measures growth inhibition as a function of drug concentration, MCBT tests 2–4 agents simultaneously at pharmacologically relevant, but fixed, concentrations to identify antibiotic combinations that kill a given bacterial isolate. Time to next PEx requiring new oral or IV antibiotics, changes in FEV1, forced vital capacity (FVC), and dyspnea scores did not differ between subjects treated with antimicrobials chosen by physicians (who presumably had access to traditional AST results from previous bacterial cultures) versus those treated with antimicrobials chosen based on MCBT [
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ]. Differences in treatment outcomes for 74 PEx treatments among 39 subjects were compared when antipseudomonal antimicrobials were selected based on conventional (planktonic) versus biofilm growth susceptibility tests [
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ]. Lung function improvements were similar in the two treatment groups; investigators noted that biofilm testing “did not lead to improved microbiological or clinical outcomes.” [
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ]

      3.1.2 Maintenance treatment of chronic bacterial infection

      Eleven articles described seven studies of antimicrobial treatments in stable patients with chronic respiratory infections; five studies addressed PICO question #1 and two addressed PICO question #2 (Fig. 1; Table 2).
      PICO Question #1: Is clinical response to antimicrobial treatment of bacterial airways infection predictable from AST results available at treatment initiation?
      One of five studies suggested a correlation between AST results and outcomes of treatment of chronic infection, while four studies did not. The one study that suggested a correlation was a prospective, randomized, open-label 8-week comparison of two different inhaled tobramycin formulations suggested that Pa isolates of differing tobramycin susceptibilities were associated with significantly different ppFEV1 responses after four weeks of treatment [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]. Among 135 subjects with only ‘susceptible’ Pa isolates (tobramycin MIC ≤4 μg/mL), mean ppFEV1 change from baseline was >8% predicted after four weeks of treatment compared with only 3.5% predicted for 24 subjects with resistant Pa isolates, with tobramycin MICs ≥16 μg/mL (P = 0.014) [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]. After four weeks off treatment, the mean ppFEV1 change from baseline decreased about 3% predicted in subjects with tobramycin-susceptible isolates and stayed about the same for subjects with higher-MIC isolates [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]. In an open-label extension, changes from baseline differed significantly between subjects with tobramycin-susceptible and -resistant isolates at only one visit (at week 32, the end of the 4th off-drug period), where subjects with resistant isolates had a mean > 2 ppFEV1 reduction from baseline while those with susceptible isolates had a mean > 4 ppFEV1 improvement (P = 0.003). Mean ppFEV1 changes from baseline were not statistically different between subjects with and without resistant isolates at any other visits [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]. In the same study, investigators reported differences in mean ppFEV1 responses for subjects with Pa isolates with tobramycin MICs ≤64 μg/mL and ≥ 128 μg/mL. Although numbers of subjects with isolates with tobramycin MICs ≥128 μg/mL were small (ranging from six to 18 subjects), mean ppFEV1 declined from baseline at five of 11 visits. At visits at weeks 28, 32, and 52, (but not at other visits) mean lung function changes were significantly lower in those with isolates with higher tobramycin MICs compared with subjects with isolates with lower tobramycin MICs [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ].
      In contrast, an analysis of data pooled from two 24-week blinded, placebo-controlled studies of inhaled tobramycin [
      • Ramsey B.W.
      • Pepe M.S.
      • Quan J.M.
      • Otto K.L.
      • Montgomery A.B.
      • Williams-Warren J.
      • et al.
      Intermittent administration of inhaled tobramycin in patients with cystic fibrosis.
      ] showed only a trend towards greater mean relative ppFEV1 improvement at week 20 for 171 tobramycin-treated subjects who had tobramycin-susceptible Pa isolates (+11% [95% CI –35%, +57%]) compared with 58 subjects with tobramycin-resistant isolates (+8% [−34%, +50%]) [
      • Ramsey B.W.
      • Pepe M.S.
      • Quan J.M.
      • Otto K.L.
      • Montgomery A.B.
      • Williams-Warren J.
      • et al.
      Intermittent administration of inhaled tobramycin in patients with cystic fibrosis.
      ]. Among tobramycin-treated subjects who rolled over into a subsequent 18-month open label, single-arm inhaled tobramycin follow-on study, neither mean ppFEV1 change nor the proportions of subjects showing net ppFEV1 improvement after 92 weeks were predicted by Pa isolate tobramycin MIC category (<16 μg/mL, 16–64 μg/mL, >64 μg/mL) [
      • Moss R.B.
      Administration of aerosolized antibiotics in cystic fibrosis patients.
      ,
      • LiPuma J.J.
      Microbiological and immunologic considerations with aerosolized drug delivery.
      ]. A similar lack of correlation between susceptibility and response was observed in two prospective, 28-day, randomized studies of inhaled antibiotics: an open-label study comparing inhaled tobramycin and inhaled colistimethate among 115 subjects [
      • Hodson M.E.
      • Gallagher C.G.
      • Govan J.R.
      A randomised clinical trial of nebulised tobramycin or colistin in cystic fibrosis.
      ,
      • Govan J.R.
      Insights into cystic fibrosis microbiology from the European tobramycin trial in cystic fibrosis.
      ] and a blinded, placebo-controlled study of inhaled levofloxacin among 151 subjects [
      • Geller D.E.
      • Flume P.A.
      • Staab D.
      • Fischer R.
      • Loutit J.S.
      • Conrad D.J.
      Levofloxacin inhalation solution (MP-376) in patients with cystic fibrosis with Pseudomonas aeruginosa.
      ], where baseline Pa isolate susceptibilities did not predict ppFEV1 change from baseline at day 28 of treatment.
      PICO Question #2: Is clinical response to antimicrobial treatment of bacterial airways infection affected by the method used to guide antimicrobial selection?
      Two studies addressed PICO question #2 in persons with CF not experiencing PEx but receiving antimicrobial maintenance treatment for chronic infection. Neither study demonstrated a significant relation between AST method and outcomes of treatment of chronic infection.
      A single-center retrospective analysis of 119 patients compared quarterly IV antimicrobial treatment outcomes before and after practice changes that reduced AST frequency by >50% [
      • Etherington C.
      • Hall M.
      • Conway S.
      • Peckham D.
      • Denton M.
      Clinical impact of reducing routine susceptibility testing in chronic Pseudomonas aeruginosa infections in cystic fibrosis.
      ]. No significant differences in median FEV1, FVC, or weight responses were observed from June to November of the year prior to the change compared to the same period of the following year, after the change, indicating that reduced AST frequency did not alter clinical outcomes [
      • Etherington C.
      • Hall M.
      • Conway S.
      • Peckham D.
      • Denton M.
      Clinical impact of reducing routine susceptibility testing in chronic Pseudomonas aeruginosa infections in cystic fibrosis.
      ].
      A randomized, prospective, multicenter pilot study to provide information for subsequent treatment of PEx compared clinical responses in stable CF patients with chronic Pa infections (i.e., not having PEx) following IV antimicrobial treatments selected based on two different susceptibility test methods: conventional (planktonic) broth microdilution and biofilm-based testing [
      • Moskowitz S.M.
      • Emerson J.C.
      • McNamara S.
      • Shell R.D.
      • Orenstein D.M.
      • Rosenbluth D.
      • et al.
      Randomized trial of biofilm testing to select antibiotics for cystic fibrosis airway infection.
      ]. Among 34 subjects with complete lung function data, those randomized to antibiotics chosen with conventional AST had a nonsignificant 70 mL better mean FEV1 response than those with antibiotics chosen based on biofilm testing (P = 0.34) [
      • Moskowitz S.M.
      • Emerson J.C.
      • McNamara S.
      • Shell R.D.
      • Orenstein D.M.
      • Rosenbluth D.
      • et al.
      Randomized trial of biofilm testing to select antibiotics for cystic fibrosis airway infection.
      ].

      4. Discussion

      Our systematic literature review has revealed scant evidence that AST is predictive of CF antimicrobial treatment response, with only three of 20 studies (two of 13 PEx [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ,
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ] and one of seven chronic maintenance studies [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ]) suggestive of a correlation. Further, those studies suggesting an AST/treatment-response association include caveats. Hubert et al. reported that Pa-isolate ceftazidime susceptibilities were associated with differences in ppFEV1 response to thrice daily ceftazidime/tobramycin PEx treatment, but similar associations were not observed for continuous ceftazidime infusion in the same patients (Fig. 2, left panel) [
      • Hubert D.
      • Le Roux E.
      • Lavrut T.
      • Wallaert B.
      • Scheid P.
      • Manach D.
      • et al.
      Continuous versus intermittent infusions of ceftazidime for treating exacerbation of cystic fibrosis.
      ]. Parkins et al. noted an inverse univariate relationship between numbers of antimicrobials used to which bacterial isolates were susceptible and proportions of PEx treatment failures, but AST results were not significant factors in multivariate models of treatment failure in the same study [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ]. Finally, significant AST-associated ppFEV1 response differences reported for inhaled tobramycin by Mazurek et al. occurred only at some study visits, but not others [
      • Mazurek H.
      • Chiron R.
      • Kucerova T.
      • Geidel C.
      • Bolbas K.
      • Chuchalin A.
      • et al.
      Long-term efficacy and safety of aerosolized tobramycin 300 mg/4 ml in cystic fibrosis.
      ].
      Poor correlation of AST with CF antimicrobial treatment response has motivated the search for alternative antimicrobial susceptibility measures for choosing antimicrobials, including MCBT [
      • Aaron S.D.
      • Vandemheen K.L.
      • Ferris W.
      • Fergusson D.
      • Tullis E.
      • Haase D.
      • et al.
      Combination antibiotic susceptibility testing to treat exacerbations of cystic fibrosis associated with multiresistant bacteria: a randomised, double-blind, controlled clinical trial.
      ] and biofilm tests [
      • Yau Y.C.
      • Ratjen F.
      • Tullis E.
      • Wilcox P.
      • Freitag A.
      • Chilvers M.
      • et al.
      Randomized controlled trial of biofilm antimicrobial susceptibility testing in cystic fibrosis patients.
      ,
      • Moskowitz S.M.
      • Emerson J.C.
      • McNamara S.
      • Shell R.D.
      • Orenstein D.M.
      • Rosenbluth D.
      • et al.
      Randomized trial of biofilm testing to select antibiotics for cystic fibrosis airway infection.
      ]. No reviewed studies, however, showed a treatment-associated benefit of these alternative methods.
      Our observations include important qualifiers, such as that conclusions drawn from studies of Pa treatment responses may not extend to other important CF bacterial opportunists. Also, AST interpretive criteria are intended to be applied to systemic, not topical (e.g., inhaled), antimicrobial treatments. Five of 20 reviewed studies were of inhaled antibiotic treatment of chronic Pa infection, where traditional systemic antimicrobial breakpoints would not be expected to predict clinical response. Further, AST/clinical response relationship analyses can only be as robust as published descriptions, which received “Low” to “Moderate” GRADE evaluations (see online supplement). Most studies provided limited information on bacterial isolate susceptibilities, identifying single isolates as susceptible, intermediate, or resistant to a given agent. In addition, most reviewed PEx treatment studies emphasized pulmonary function responses, but signs and symptoms of infection are important motivators of PEx treatment and symptom reduction has been reported to be a higher priority for patients/families than lung function recovery [
      • Heltshe S.L.
      • West N.E.
      • VanDevanter D.R.
      • Sanders D.B.
      • Beckett V.V.
      • Flume P.A.
      • et al.
      STOP study group. Study design considerations for the standardized treatment of pulmonary exacerbations 2 (STOP2): a trial to compare intravenous antibiotic treatment durations in CF.
      ]. Also, studies of ppFEV1 response, which has a relatively large variance, were likely underpowered to detect significant response differences [
      • VanDevanter D.R.
      • Heltshe S.L.
      • Spahr J.
      • Beckett V.V.
      • Daines C.L.
      • Dasenbrook E.C.
      • et al.
      STOP Study Group. Rationalizing endpoints for prospective studies of pulmonary exacerbation treatment response in cystic fibrosis.
      ]. Finally, use of ppFEV1 as an outcome variable was inconsistent, complicating comparisons. One study defined any positive ppFEV1 change from admission as PEx treatment success [
      • Wolter J.M.
      • Bowler S.D.
      • McCormack J.G.
      Are antipseudomonal antibiotics really beneficial in acute respiratory exacerbations of cystic fibrosis?.
      ], another used >5 ppFEV1 improvement to define treatment success [
      • Smith A.L.
      • Fiel S.B.
      • Mayer-Hamblett N.
      • Ramsey B.
      • Burns J.L.
      Susceptibility testing of Pseudomonas aeruginosa isolates and clinical response to parenteral antibiotic administration: lack of association in cystic fibrosis.
      ], and two studies defined treatment failure as achieving <90% of a prior year's best ppFEV1 measure [
      • Parkins M.D.
      • Rendall J.C.
      • Elborn J.S.
      Incidence and risk factors for pulmonary exacerbation treatment failures in patients with cystic fibrosis chronically infected with Pseudomonas aeruginosa.
      ,
      • Lam J.C.
      • Somayaji R.
      • Surette M.G.
      • Rabin H.R.
      • Parkins M.D.
      Reduction in Pseudomonas aeruginosa sputum density during a cystic fibrosis pulmonary exacerbation does not predict clinical response.
      ].
      There are important limitations to our analysis, including that our search strategy utilized citation titles and abstracts: if important susceptibility-related differences in outcomes were not mentioned in titles or abstracts, an article was unlikely to advance beyond secondary screening. Study design and reporting variability made it impossible to combine small studies into a larger dataset for meta-analyses, rendering our analyses largely descriptive. PEx management involves more than antimicrobial administration [
      Treatment of pulmonary exacerbation of cystic fibrosis.
      ]; airway clearance, nutritional support, and psychosocial support available to admitted patients may also contribute to FEV1 response. Importantly, scant attention has been paid in past studies to safety risks that may be associated with using AST to guide antimicrobial choices: when P. aeruginosa becomes ‘resistant’ in vitro to commonly used antipseudomonals, clinicians may be tempted to prescribe treatments that have less-favorable risk profiles and with which they may have less familiarity. Finally, there is good reason to believe that direct comparisons of antimicrobial treatment responses between patients with ‘susceptible’ versus ‘resistant’ bacterial isolates are confounded by differences in patient age, disease aggressiveness and lung disease stage (Fig. 3). In CF, most new Pa acquisitions are wild-type, susceptible organisms [
      • VanDevanter D.R.
      • Van Dalfsen J.M.
      • Burns J.L.
      • Mayer-Hamblett N.
      In vitro antibiotic susceptibility of initial Pseudomonas aeruginosa isolates from US cystic fibrosis patients.
      ], with selection for antimicrobial resistance resulting from successive treatments, presumably due to repeated clinical presentation of signs and symptoms of respiratory tract infection. Thus, antimicrobial resistance may act as a biomarker for patients with both more aggressive and advanced airway disease [
      • Lechtzin N.
      • John M.
      • Irizarry R.
      • Merlo C.
      • Diette G.B.
      • Boyle M.P.
      Outcomes of adults with cystic fibrosis infected with antibiotic-resistant Pseudomonas aeruginosa.
      ] who may not respond to antimicrobial treatment in the same manner as those with less aggressive and/or less advanced disease [
      • VanDevanter D.R.
      • Konstan M.W.
      CF drug developers: victims of our own Success.
      ,
      • VanDevanter D.R.
      • O'Riordan M.A.
      • Blumer J.L.
      • Konstan M.W.
      Assessing time to pulmonary function benefit following antibiotic treatment of acute cystic fibrosis exacerbations.
      ], regardless of bacterial isolate susceptibility. This confounding, taken together with known differences between bacterial growth/metabolism in the lab and in the lung [
      • Prince A.S.
      Biofilms, antimicrobial resistance, and airway infection.
      ], sampling constraints introduced by picking colonies of heterogeneous bacterial populations [
      • Foweraker J.E.
      • Laughton C.R.
      • Brown D.F.
      • Bilton D.
      Phenotypic variability of Pseudomonas aeruginosa in sputa from patients with acute infective exacerbation of cystic fibrosis and its impact on the validity of antimicrobial susceptibility testing.
      ], established and yet to be recognized interspecies effects on bacterial susceptibilities in complex airway infections [
      • Hoffman L.R.
      • Déziel E.
      • D'Argenio D.A.
      • et al.
      Selection for Staphylococcus aureus small-colony variants due to growth in the presence of Pseudomonas aeruginosa.
      ], and the presence of bacterial persister strains in airway infections [
      • Mulcahy L.R.
      • Burns J.L.
      • Lory S.
      • Lewis K.
      Emergence of Pseudomonas aeruginosa strains producing high levels of persister cells in patients with cystic fibrosis.
      ] all complicate the interpretation of current AST test results in CF.
      Fig. 3
      Fig. 3A directed acyclic graph (DAG) of a proposed relationship between CF bacterial isolate antimicrobial susceptibility and antimicrobial clinical response. A “back door path” suggestive of confounding [
      • Lederer D.J.
      • Bell S.C.
      • Branson R.D.
      • et al.
      Control of confounding and reporting of results in causal inference studies: guidance for authors from editors of respiratory, sleep, and critical care.
      ] exists between an individual's bacterial isolate susceptibilities and their potential for clinical response to antimicrobial treatment. The probability of having bacterial isolates with reduced susceptibility is influenced by an individual's history of antimicrobial treatment [
      • VanDevanter D.R.
      • Van Dalfsen J.M.
      • Burns J.L.
      • Mayer-Hamblett N.
      In vitro antibiotic susceptibility of initial Pseudomonas aeruginosa isolates from US cystic fibrosis patients.
      ], which in turn is influenced by both their age and the aggressiveness of their CF lung disease [
      • Zhao J.
      • Schloss P.D.
      • Kalikin L.M.
      • Carmody L.A.
      • Foster B.K.
      • Petrosino J.F.
      • et al.
      Decade-long bacterial community dynamics in cystic fibrosis airways.
      ]. The nature of an individual's FEV1 response to antimicrobial treatment is influenced by their age as well as their lung disease stage [
      • VanDevanter D.R.
      • Konstan M.W.
      CF drug developers: victims of our own Success.
      ,
      • VanDevanter D.R.
      • O'Riordan M.A.
      • Blumer J.L.
      • Konstan M.W.
      Assessing time to pulmonary function benefit following antibiotic treatment of acute cystic fibrosis exacerbations.
      ], which is in turn influenced by their lung disease phenotype [
      • Konstan M.W.
      • Wagener J.S.
      • VanDevanter D.R.
      Characterizing aggressiveness and predicting future progression of CF lung Disease.
      ]. In addition, it has been shown that antimicrobial treatment history markedly affects the diversity of the airway microbial community [
      • Zhao J.
      • Schloss P.D.
      • Kalikin L.M.
      • Carmody L.A.
      • Foster B.K.
      • Petrosino J.F.
      • et al.
      Decade-long bacterial community dynamics in cystic fibrosis airways.
      ], which may in turn also influence antimicrobial response.
      Our findings highlight a lack of proven benefit for a routine CF practice and suggest that the CF community consider reducing the frequency of routine susceptibility testing (as was studied by Etherington et al. [
      • Etherington C.
      • Hall M.
      • Conway S.
      • Peckham D.
      • Denton M.
      Clinical impact of reducing routine susceptibility testing in chronic Pseudomonas aeruginosa infections in cystic fibrosis.
      ]). Our results indicate a clear need for more robust evaluation of antimicrobial treatment-associated outcomes as well as prioritization of innovative research and development in the area of antimicrobial utilization. Prospective clinical trials are difficult to conduct in this area due to the ethics of randomizing patients to antimicrobial regimens to which their bacterial isolates are ‘resistant’ according to AST. An opportunity remains, however, to conduct large, comprehensive retrospective analyses of relationships between bacterial isolate susceptibility, antimicrobial treatment, and treatment response using care center records or patient registries. Such analyses may substantially improve understanding of CF antimicrobial treatment outcomes, particularly if addressing possible confounders such as patient age, disease aggressiveness, and disease stage, and may serve to temper unnecessary resource utilization and improve treatment outcomes in individuals with CF.

      Acknowledgements

      This work was funded by the European Cystic Fibrosis Society , Cystic Fibrosis Foundation , Cystic Fibrosis Trust , Cystic Fibrosis Canada , and Cystic Fibrosis Australia .

      Appendix A. Supplementary data

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