Routine use of Gen-Probe Amplified Mycobacterium Tuberculosis Direct (MTD) test for detection of Mycobacterium tuberculosis with smear-positive and smear-negative specimens.

Setting: Nucleic acid amplification tests, such as the Amplified Mycobacterium Tuberculosis Direct (MTD) Test, may improve early diagnosis of tuberculosis when used in combination with acid-fast bacilli smear examination with a similar turnaround time.

Objective: To evaluate the routine use of MTD in respiratory and non-respiratory samples; to investigate the improvement of MTD specificity and positive predictive value by defining an equivocal zone for result interpretation.

Design: MTD was performed according to the instructions supplied by the manufacturer. An equivocal zone was included for interpretation of results. Discordant results with culture were resolved by incorporating clinical data and multiple specimen analysis.

Results: The overall sensitivities, specificities, and positive and negative predictive values for respiratory specimens (n = 3308) were 90.8, 99.9, 99.1, and 99.2%, respectively. With extra-pulmonary specimens (n = 1350) those values were 67.4, 99.9, 98.2, and 97.9%, respectively. By implementing an equivocal zone, the specificity and positive predictive value of MTD were improved (from 99.1% and 88.6% to 99.9% and 98.9% respectively) without significantly altering other performance characteristics.

Conclusions: Amplification assays cannot yet replace the conventional diagnostic techniques. Nevertheless, MTD is a reliable method for the direct detection of M. tuberculosis in clinical specimens. The number of false-positive results can be limited by defining an equivocal zone.