Quantitative susceptibility test methods in major United States medical centers

To assess the current trend toward performance of routine quantitative susceptibility test methods, we surveyed microbiology laboratories in 41 major U.S. medical centers. Agar dilution and microdilution tests were used primarily by those laboratories routinely performing numerous minimal inhibitory concentration tests and few or no minimal bactericidal concentration tests. Macrotube dilution tests were used by laboratories performing relatively few minimal inhibitory concentration tests, but routinely performing minimal bactericidal concentration tests. Major methodological variations were reported by the surveyed laboratories and including: (i) preparation and storage of antimicrobial solutions, (ii) standardization and concentration of the test inoculum, (iii) interpretation of minimal inhibitory concentration endpoints, and (iv) determination of the antibiotic concentration required to be bactericidal. The results of this survey indicate that significant variations in test methods exist, even though quantitative susceptibility tests are commonly used.