Multisite Reproducibility of the Broth Microdilution Method for Susceptibility Testing of Nocardia Species

Antimicrobial susceptibility testing (AST) of clinical isolates ofNocardia is recommended to detect resistance to commonly used antimicrobial agents; such testing is complicated by difficulties in inoculum preparation and test interpretation. In this study, six laboratories performed repetitive broth microdilution testing on single strains ofNocardia brasiliensis ,Nocardia cyriacigeorgica ,Nocardia farcinica ,Nocardia nova , andNocardia wallacei . For each isolate, a total of 30 microdilution panels from three different lots were tested at most sites. The goal of the study was to determine the inter- and intralaboratory reproducibility of susceptibility testing of this group of isolates. Acceptable agreement (>90% agreement at ±1 dilution of the MIC mode) was found for amikacin, ciprofloxacin, clarithromycin, and moxifloxacin. After eliminating MIC values from single laboratories whose results showed the greatest deviation from those of the remaining laboratories, acceptable agreement was also found for amoxicillin-clavulanic acid, linezolid, minocycline, and tobramycin. Results showed unsatisfactory reproducibility of broth microdilution testing of ceftriaxone withN. cyriacigeorgica andN. wallacei , tigecycline withN. brasiliensis andN. cyriacigeorgica , and sulfonamides withN. farcinica andN. wallacei. N. nova ATCC BAA-2227 is proposed as a quality control organism for AST ofNocardia sp., and the use of a disk diffusion test for sulfisoxazole is proposed as a check of the adequacy of the inoculum and to confirm sulfonamide MIC results.