Evaluation of the KOH test and the antibiotic disk test in routine clinical anaerobic bacteriology

We have evaluated the KOH test, the antibiotic disk identification test, and the Gram stain reaction for the preliminary grouping of gram-positive and gram-negative anaerobes and have assessed the value of erythromycin 60-micrograms-disk resistance as a predictive index of clindamycin resistance among anaerobes. By testing 931 clinical isolates, 281 gram positive and 650 gram negative, with the KOH test and vancomycin 5-micrograms-disk test, we obtained the following parameters: sensitivity, 89.7 and 97.1%; specificity, 97.5 and 98.3%; positive predictive value, 80.4 and 98.7%; and efficiency, 92.1 and 98% for the KOH test and the vancomycin test, respectively. The KOH reaction incorrectly grouped 42 of 97 Bacteroides bivius and 12 of 50 pigmented Bacteroides strains. The vancomycin test correctly identified 63 of 67 gram-negative strains that had given a negative KOH reaction. The erythromycin disk result correctly predicted clindamycin resistance in gram-negative isolates but had a sensitivity of 85.7%, a specificity of 92.4%, and a positive predictive value of 42.8% for gram-positive isolates. Therefore, the use of these preliminary identification tests can assist in the correct grouping of anaerobes and accurately predict significant clindamycin resistance in gram-negative anaerobic bacteria.