Evaluation of reference dilution test methods for antimicrobial susceptibility testing of Pseudomonas aeruginosa strains isolated from patients with cystic fibrosis

The development of multidrug-resistant Pseudomonas aeruginosa in patients w ith cystic fibrosis (CF) is most likely a consequence of increasing life ex pectancy and more prolonged exposure to antibiotics. The optimal method for antibiotic susceptibility testing of CF strains, particularly mucoid P. ae ruginosa strains, is unknown. Antimicrobial susceptibilities of 48 CF strai ns (25 mucoid) and 50 non-CF strains to 12 anti;Pseudomonas agents were tes ted by both agar dilution and commercially custom-prepared broth microdilut ion plates (PML Microbiologicals, Portland, Oreg,) in three laboratories si multaneously to determine if broth microdilution could substitute for agar dilution as the reference method in subsequent studies. Comparison of MICs generated by agar dilution and broth microdilution demonstrated correlation coefficients (r) exceeding 0.85 for all agents tested; correlation was exc ellent for aminoglycosides (r greater than or equal to 0.92) and very good for beta-lactam agents including agents paired with a beta-lactamase inhibi tor (r greater than or equal to 0.87) and for ciprofloxacin (r = 0.86), Cor relation was not improved by 48-h readings, but correlation between 24- and 48-h readings ranged between 0.91 and 0.98 for both methods. Interlaborato ry variations were minimal, as the percentage of acceptable variations was 94% for both methods, and serious discords were infrequent (<2% of comparis ons), However, CF strains were more likely to have serious discords than we re non-CF strains (P < 0.0001), although mucoid strains were not more likel y to have serious discords than were nonmucoid strains. In this study, MICs determined by custom-prepared broth microdilution compared favorably with MICs determined by agar dilution. Thus, this broth microdilution assay can serve as a reference method and facilitate future studies to determine the optimal method for antibiotic susceptibility testing of CF strains.