AMPLICOR™ MTB polymerase chain reaction test for identification of Mycobacterium tuberculosis in positive difco ESP II broth cultures

Abstract

The reliability of the Roche AMPLICOR™ polymerase chain reaction (PCR) Mycobacterium tuberculosis (MTB) test for identification of M. tuberculosis complex (MTBC) in Difco ESP II broth cultures was evaluated by testing aliquots from all cultures, regardless of specimen type, that were signaled positive by the ESP instrument. Polymerase chain reaction results were compared to those obtained by usual laboratory protocol, whereby MTBC was identified by DNA probe (Gen-Probe, Inc., San Diego, California, USA), testing sediment from broth samples or colonies on a solid medium. Of the 242 signal-positive ESP II cultures (from 182 patients) evaluated, 98 (40.5%) contained mycobacteria, including 26 MTBC. On initial testing, 27 samples were positive by PCR; 22 of these were MTBC culture positive, and 5 were culture negative for mycobacteria. The sensitivity, specificity, and positive and negative predictive values of PCR were 84.6, 97.7, 81.5, and 98.1%, respectively. Based on review of the medical records of the patients for whom PCR and culture results were discrepant, one PCR-positive culture negative was a true PCR positive; thus the actual sensitivity, specificity, and positive and negative predictive values of PCR were 85.2, 98.1, 85.2, and 98.1%. Two of the PCR-negative, MTBC culture-positive samples were blood specimens collected in an Isolator tube; and these tubes contain sodium polyanetholsulfonate, which seems to inhibit PCR. If blood specimens are excluded from analysis, the sensitivity, specificity, and positive and negative predictive values for PCR are 92.0, 97.6, 85.2, and 98.8%, respectively. The mean times from specimen receipt to identification of MTBC were 19 (± 2) days (range, 6 to 39 days) for ESP II plus AMPLICOR™ MTB PCR and 25 (± 2) days (range, 8 to 42 days) for DNA probe testing of sediment from broth or colonies on solid media (p < .05). These data indicate that the AMPLICOR™ MTB PCR test is a rapid, reliable method for identification of MTBC in positive ESP II cultures, excluding blood specimens collected in Isolator tubes.