Broth-disk elution tests to predict the susceptibility of anaerobic bacteria to the ampicillin-sulbactam combination.

The ampicillin-sulbactam Wilkins-Chalgren agar dilution MICs (2 ratios) were compared with the results of broth-disk elution (BDE) tests using various numbers of disks corresponding to elution breakpoints of less than or equal to 8:8 micrograms/ml, less than or equal to 16:8 micrograms/ml, and less than or equal to 16:16 micrograms/ml. This study showed that a 2:1 ratio MIC test and a 16:8 micrograms/ml BDE breakpoint were best for anaerobic organisms; a recommendation consistent with susceptibility testing criteria for rapidly growing aerobic species. The addition of four ampicillin disks (10 micrograms) and four ampicillin-sulbactam discs (10:10 micrograms) to 5 ml of thioglycolate broth was recommended. This test would achieve greater than 99% comparative accuracy to MICs determined with the reference National Committee for Clinical Laboratory Standards agar dilution method.