Enhanced mycobacterium tuberculosis direct test for detection of M. tuberculosis complex in positive ESP II broth cultures of nonrespiratory specimens

Abstract

The reliability of the Gen-Probe enhanced Mycobacterium Tuberculosis Direct Test (E-MTD) for identification of M. tuberculosis complex (MTBC) in ESP II broth cultures of nonrespiratory specimens was evaluated by testing aliquots from 262 signal-positive bottles. Cultures of blood were excluded from analysis because of the high rate (13/18 [72.2%]) of equivocal results for aliquots from cultures that were negative for MTBC. E-MTD results were compared to those obtained by usual laboratory protocol, whereby MTBC was identified by DNA probe (Gen-Probe, Inc.), testing colonies on a solid medium. For the first 112 cultures, from which 34 mycobacteria (including 8 MTBC) were recovered, both fresh and frozen aliquots were tested. E-MTD results on both fresh and frozen aliquots agreed with identification by usual protocol in all cases; so for the remaining 150 cultures, only frozen aliquots were tested. Of the total 262 ESP II cultures (158 patients) evaluated, 60 (22.9%) grew mycobacteria, including 17 MTBC. Of these 17 aliquots, 16 were E-MTD positive, as was one culture that was negative for mycobacteria. The latter culture was from a patient who had a specimen collected the following day from which MTBC was isolated; therefore, the E-MTD result was considered a true positive. Sensitivity, specificity, and positive and negative predictive values of E-MTD were 94.1, 100, 100, and 99.6%, respectively. The mean times (± SEM) from specimen receipt to identification of MTBC were 17 (±2) days (range, 5–30 days) for ESP II plus E-MTD and 19 (±1) days (range, 6–28 days) for the usual protocol (Student’s t test, P, not significant). These data indicate that the performance of E-MTD for identification of MTBC in fresh or frozen aliquots of broth from positive ESP II cultures of nonrespiratory specimens, excluding blood, is excellent. However, because the E-MTD did not significantly reduce time to identification of MTBC in these cultures, limiting testing to those types of specimens most likely to yield MTBC is reasonable.