AMPICILLIN-SULBACTAM SUSCEPTIBILITY TESTING CRITERIA

In vitro studies in five different medical centers documented the susc eptibility of 2,440 consecutive isolates of the Enterobacteriaceae aga inst ampicillin-sulbactam disks of different potencies. For determinat ion of MICs, both 2:1 or 1:1 ratios were used as long as the concentra tions 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/m l for the susceptible category. Disks containing 10 mug of ampicillin and 10 mug of sulbactam are still to be preferred with interpretive cr iteria 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 m ug/ml or greater-than-or-equal-to 32/32 mug/ml). The reliability of th e disk test actually diminished when the amount of sulbactam in the di sk was increased.