Diagnostic value of an amplification method (Gen-Probe) compared with that of culture for diagnosis of tuberculosis

Abstract

Five hundred fifty respiratory and nonrespiratory specimens from 340 patients were analyzed by comparing the Gen-Probe Amplified Mycobacterium Tuberculosis Direct Test (MTD) with conventional culture, which was the method of reference, for the detection of the Mycobacterium tuberculosis complex. After resolution of discrepant results by retesting the samples and reviewing the patients' clinical histories, a total of 60 respiratory specimens were MTD and culture positive, 347 were MTD and culture negative, 4 were MTD positive and culture negative, and 1 was MTD negative and culture positive. This results in a sensitivity of 98.4%, a specificity of 98.9%, and positive and negative predictive values of 93.8 and 99.7%, respectively. Repeatedly, clinicians asked to test specimens of nonpulmonary origin by MTD. Although, MTD is not approved for use with nonrespiratory specimens, the following results were shown. Sixty-one pleural exudate specimens showed disappointing results (sensitivity, 20%). However, MTD performed well with another 77 nonrespiratory specimens; 17 samples were positive and 57 samples were negative by both MTD and culture. No false-negative results were found by MTD. Three MTD-positive, culture-negative specimens had high sample relative light unit/cutoff relative light unit ratios, strongly suggesting true tuberculosis. Positive microscopy and positive culture with MTD-negative results occurred 12 times. Those cultures showed atypical mycobacteria 11 times and Actinomyces species once. The stability of the reagents in the MTD kit was also assessed by testing reagents, including the enzyme mixture, kept at -70 degrees C for at least 6 months. No loss of activity was seen.