Disk elution method for MICs and MBCs.

A MIC macrodilution broth method in which routine antimicrobial susceptibility testing disks were used as a source of antibiotic was compared with the M7A reference method of the National Committee for Clinical Laboratory Standards for accuracy and reproducibility over five test runs. Gentamicin, piperacillin, and oxacillin were tested by using American Type Culture Collection quality control strains and laboratory isolates for which the MIC of each agent was known. An optimum time for elution of 1 h was first determined by timed elution in broth, confirming levels determined by the TDX method (Abbott, North Chicago, Ill.) for gentamicin and bioassay for piperacillin and oxacillin. MICs determined by the disk elution method demonstrated a reproducibility rate of 97% as well as 97% agreement with the reference method to within +/- 1 dilution; MBCs showed 93% agreement. The disk elution method is reproducible and provides a reliable test system that can be used by any laboratory in which disk susceptibility testing is performed.