AMPICILLIN-SULBACTAM SUSCEPTIBILITY TESTING CRITERIA

In vitro studies in five different medical centers documented the susceptibility of 2,440 consecutive isolates of the Enterobacteriaceae against ampicillin-sulbactam disks of different potencies. For determination of MICs, both 2:1 or 1:1 ratios were used as long as the concentrations of sulbactam at the breakpoints remained the same, i.e. MIC less-than-or-equal-to 16/8.0 mug/ml or less-than-or-equal-to 8.0/8.0 mug/ml for the susceptible category. Disks containing 10 mug of ampicillin and 10 mug of sulbactam are still to be preferred with interpretive criteria of greater-than-or-equal-to 15 mm for susceptible and less-than-or-equal-to 11 mm for resistant (MIC greater-than-or-equal-to 64/32 mug/ml or greater-than-or-equal-to 32/32 mug/ml). The reliability of the disk test actually diminished when the amount of sulbactam in the disk was increased.