[Evaluation of the MTD-2 test for the direct detection of Mycobacterium tuberculosis in specimens]

To evaluate the utility of the Amplified Mycobacterium Tuberculosis Direct Test (MTD-2, Gen-Probe) in the microbiological diagnosis of tuberculosis.

We evaluated the results obtained in 146 specimens with the MTD-2 test, together with those of the culture and smears. The MTD-2 test was performed on all the smear positives specimens (n=47), on the smear-negative specimens, when the test was demanded (n=19), and in other smear-negative specimens previously selected, according to the clinical history of the patient (n=80). We considered real cases of tuberculosis, those that were culture positive for Mycobacterium tuberculosis and those that were specifically treated.

The overall sensitivity, specificity, positive and negative predictive values for the MTD test were: 95, 76, 71, and 96%, for the culture; and 84, 100, 100 and 90% and 75, 94, 89 and 86% for the smears, respectively. In smear positive specimens, the test showed a great specificity, and differentiated M. tuberculosis from other mycobacteria. In the smear negatives, the sensitivity of the test was low and so was the positive predictive value, especially in series performed with a high work load.

Data from our study show that the MTD-2 test is a reliable method for rapid diagnosis of tuberculosis in smear positive specimens. However, due to its low sensitivity and positive predictive value, it is not recommended in the routine diagnosis of tuberculosis. Also, for this reason, whenever a positive result is obtained with a smear negative specimen, the result needs to be confirmed with another specimen.