Xpert MTB-RIF validation study from Tanzania

Polly Clayden, HIV i-Base

The Xpert MTB-RIF assay (Xpert, described in detail in the article below) gained a lot of attention at this meeting.

This is a cartridge-based, real-time PCR test with automated sample processing, amplification, detection of M. tuberculosis and resistance to rifampicin (RIF).

Andrea Rachlow presented data from an evaluation study of this test performed in Tanzania.

This study enrolled 292 consecutive symptomatic patients. These patients were classified as TB positive or negative following results of sputum smear, culture on solid and liquid media on three different sputum samples (plus chest X-ray and HIV test), and sustained recovery after two months follow-up.

Stored samples were then tested with the Xpert (three frozen, untreated sputum samples per patient).

The investigators reported, that of 69 culture-positive TB cases, Xpert detected 88.4% (95% CI 78– 95%). Sensitivity was notably different between smear-positive and only culture-positive patients, with sensitivities of 98% and 61% respectively.

Among all TB negative patients, Xpert detected one positive result (99% specificity). One of the samples from 45 patients that were culture-positive for non-tuberculous mycobacteria (NTM) also tested positive with Xpert.

Additionally, the test performed well in HIV-positive patients (n=50) with 88% sensitivity and 89% specificity.

The investigators noted that the test is easy to use and the short time to a result could mean avoidance of loss to follow up during the diagnostic process, which could result in a 5-15% decrease in TB deaths worldwide.

They added that further studies are required to confirm the tests performance on fresh sputum samples and on other clinical material.


Rachlow A et al. Detection of TB using the Cepheid Xpert MTB/RIF Assay: a clinical validation study from Tanzania. The International Journal of Tuberculosis and Lung Health. FA-101098-13

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