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Mycobacteria Tuberculosis PCR

Traditional testing for pulmonary tuberculosis (MTB) includes AFB smear & culture. Unfortunately, AFB culture can take up to 6 weeks for results, due to the slow-growing nature of the organisms. Although more timely with regard to results, AFB smears lack sensitivity, and may be negative in up to 50% of culture-confirmed MTB infections. Molecular testing, that improves time to detection of MTB and facilitates treatment and determination of isolation requirements, has been available for several years.

The CDC recommends that molecular testing be performed on at least one respiratory specimen from patients with suspected pulmonary TB for whom the result would alter management or infection control activities (MMWR 2013;62:821-827).

In 2014, a new rapid qualitative PCR (Cepheid Xpert MTB) became available that allows direct testing of sputum samples for MTB and simultaneously detects the rifampin-resistance rpoB gene. (Boehme et al. NEJM, Sept 2010). In 2015, CDC published updated performance characteristics for Cepheid Xpert MTB (MMWR 64(07);193). Based on results of a multicenter international study, results from one or two sputum specimens are as reliable as results obtained when multiple specimens are submitted.

A single Xpert MTB PCR test detected 97% of AFB smear-positive, culture-positive MTB patients, while two serial Xpert MTB PCR tests detected 100%. The negative predictive value of a single negative Xpert MTB PCR result was 99.6% calculated for United States MTB disease prevalence. Finally, a single Xpert MTB PCR assay detected 55% of AFB smear-negative, culture-positive specimens, while two successive tests improved recovery to 69%.

Molecular testing does not replace the need for traditional testing, hence all sputum specimens received for MTB PCR should have AFB smear and culture ordered as well.

Results are reported as ‘MTB detected’ or ‘MTB not detected’ and ‘rifampin resistance detected’ or ‘rifampin resistance not detected’.

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