Direct identification and susceptibility testing by the AutoMicrobic system of gram-negative bacilli from urine specimens

Abstract

A total of 1,800 urine specimens were screened by Gram stain to detect bacteriuria. Pellets of bacteria were obtained by centrifuging specimens containing greater than or equal to 1 gram-negative bacillus of a single morphological type per oil immersion field. Direct susceptibility tests and identifications were performed from pellets by using the AutoMicrobic system (AMS). Results were compared with culture results by routine AMS methods. Of the 145 specimens showing only gram-negative bacilli on Gram stain, 113 grew greater than or equal to 10(5) CFU of a single species per ml. Compared with routine AMS identifications, the direct method correctly identified 105 (92.9%) of the isolates. Identifications were available within 8 h for 77% of the isolates. When compared by MICs, 93.2% of the direct susceptibility test results agreed with routine AMS results within one twofold dilution. Comparisons by category call indicated that overall complete and essential agreements were 89.9 and 97.8%, respectively, with 1.0% very major, 1.0% major, and 8.1% minor errors. Cefamandole and cephalothin had the lowest correlations by both comparisons. Within 8 h, susceptibility results were available for 94.3% of the isolates. This method offers the advantage of rapid detection, prompt processing, and earlier reporting of complete results for positive urine specimens.