The categorization of prior antibiotic use: Impact on the identification of risk factors for drug resistance in case control studies

Background Analytic approaches to the identification of risk factors for the development of drug resistance vary and may affect study results. Using fluroquinolone-resistant Pseudomonas aeruginosa (FQRPA) and imipenem-resistant P. aeruginosa as models (IRPA), we aimed to examine the effect of different approaches to classification of prior antibiotic use (i.e., class versus spectrum) on the identification of risk factors for antibiotic resistance. Methods Case–control studies to identify risk factors for FQRPA and IRPA were performed. In each, two analytic models were utilized. In the first, prior antibiotic use was classified by class, and in the other, prior antibiotic use was classified by spectrum of activity. Risk factors identified by the two models were compared qualitatively for each resistant organism. Results 879 isolates of P. aeruginosa were included in the case–control studies. Risk factors for FQRPA and IRPA identified in multivariable analyses differed based on which method of classification of prior antibiotic use was utilized. Conclusions The identification risk factors for the development of drug-resistant organisms could depend on the method of classification of prior antibiotic use. In studies of risk factors for resistant infections, the approach to classification of prior antibiotic use should be clearly stated and justified.