Serology as a diagnostic tool for predicting initialPseudomonas aeruginosa acquisition in childrenwith cystic fibrosis

Rationale: Pseudomonas aeruginosa (Pa) serology could potentially be a useful adjunct to respiratory culture methods for the detection of initial or early Pa infection in patients with cystic fibrosis (CF). Objective: To evaluate the utility of Pa serology to predict Pa isolation from respiratory (generally oropharyngeal) cultures in the subsequent 6 or 12. months among young children with CF from whom Pa had never been previously cultured. Pa serology was also evaluated in a group of healthy controls. Methods: Children≤12years of age without prior isolation of Pa from respiratory cultures participating in the Early Pseudomonal Infection Control EPIC Observational Study (EPIC OBS) had annual serum samples for measurement of antibodies against alkaline protease, elastase and exotoxin A using a commercial kit; controls had a single serum sample. Logistic regression with generalized estimating equations was used to characterize associations between log₁₀ serum antibody titers and first isolation of Pa from a respiratory culture within the subsequent 6 or 12months, with adjustment for sex and age. Receiver operating characteristic curves were used to optimize antibody titer cutpoints by age group. The diagnostic properties of each antibody were estimated using these optimized cutpoints. Results: Pa serology was evaluated in 582 children with CF (2084 serum samples) and 94 healthy controls. There was substantial overlap between serum antibody titers among controls, CF patients who did not acquire Pa (N = 261) and CF patients who did acquire Pa (N = 321). The maximum positive predictive value for first Pa positive culture within the ensuing 6. months was 76.2% and maximum negative predictive value was 72.1% for any antigen or combination of antigens; values were similar for 12. months. Conclusions: Pa serology does not appear useful for predicting first Pa positive oropharyngeal culture among young CF patients.