Mycobacterium tuberculosis complex Direct RNA Detection from Clinical Specimens
By Xiaotian Zheng, PhD, DLS Microbiology Director, June 1999
Rapid and accurate detection of Mycobacterium tuberculosis complex (MTBc) is a key aspect of effective tuberculosis treatment and control. The Amplified Mycobacterium tuberculosis complex Direct Test (Gen-Probe) utilizes transcription mediated amplification to detect MTB complex ribosomal RNA. This test is very sensitive, specific, and can be performed within 5 hours. Although it does not differentiate among members of the MTB complex, i.e., M. tuberculosis, M. bovis, M. bovis BCG, M africanum, and M. microti , the isolation of the last 4 organisms is very rare.
The test is currently approved by FDA for AFB smear-positive respiratory specimens from untreated patients (sensitivity = 98%, specificity = 96.8%). However, studies have shown that it will also benefit patients with high suspicion of tuberculosis, but with negative AFB smear results. A positive result is strongly suggestive for the diagnosis of tuberculosis if the clinical history and radiographic finding are consistent. This test should not replace AFB smear and culture since they will determine if mycobacterium other than tuberculosis complex are present in addition to MTB complex and the culture is necessary to perform antimycobacterial susceptibility testing.
This test has not been studied for use with specimens from patients being treated with antituberculous agents to determine bacteriologic cure or to monitor response to such therapy, and positive results may be caused by the presence of MTBc RNA from nonviable organisms.
DLS evaluated the test using 805 specimens. All 24 specimens with positive AFB smears and positive cultures (MTB) gave positive results for the MTB direct RNA amplification. Of the 16 AFB smear negative but culture MTB positive specimens, 9 were MTB positive by the initial rapid test and another 5 positive by repeat. This confirms the high positive predictive value of the test. The initial negative results on smear negative but culture positive specimens are due to low inoculum and uneven distribution of the organisms in these specimens.
There were no false-positive results observed among the 157 smear and culture-negative specimens and among the 84 specimens cultured positive for AFBs other than MTB.
- Request the test when MTB infection is highly suspected;
- Always have AFB smear and culture done;
- The sensitivity for some non-respiratory specimens (e.g. tissue) can be low, and repeat testing may be indicated if the first sample test result is negative.
- This test is not for monitoring antibiotic therapy;
- The test can not be performed on bloody specimens because of interference.
Dr. Xiaotian Zheng, Phone: 547-4170. (589-5237, after July 25, 1999) Dr. Kirk Hirata, Phone: 547-4271.
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