A NEW RELIABLE ASSAY FOR TB DIAGNOSIS 

A Cochrane Review shows that the rapid, automated XpertMTB/RIF assay is a sensitive and specific test for tuberculosis (TB). It is effective when used as an initial diagnostic test for TB as well as for rifampicin-resistant TB.

The meta-analysis was authored by Karen R. Steingart, MD, MPH, from the University of Washington in Seattle, and colleagues and published online January 31 in the Cochrane Library. The review included 18 studies involving a total of 7816 people. Two of the studies were multicenter international studies, and most took place in low- and middle-income countries.

The assay is used to analyze the sputum of patients who are suspected of having TB and multidrug-resistant TB (MDR-TB), with or without HIV. Its use is endorsed by the World Health Organization.

Sensitivity and Specificity

In their review, the authors estimated median pooled sensitivity and specificity for the assay, as well as the 95% credible intervals (CrI).

Xpert was found to have a pooled sensitivity of 88% (95% CrI, 83% - 92%) and pooled specificity of 98% (95% CrI, 97% - 99%). For rifampicin resistance detection, the pooled sensitivity was 94% (95% CrI, 87% - 97%) and the pooled specificity was 98% (95% CrI, 97% - 99%). As an add-on test after a negative smear microscopy, the assay had a pooled sensitivity of 67% (95% CrI, 58% - 74%) and a pooled specificity 98% (CrI 95%, 97% - 99%).

The authors conclude that the sensitivity and specificity of Xpert qualifies it as an initial diagnostic test in people suspected of having TB, MDR-TB, or HIV-associated TB. It may also be valuable as an add-on test for patients who are smear-negative by microscopy. The researchers note that any test that is positive for rifampicin resistance should be considered in the context of MDR-TB rates in the given setting. The authors also point out that the assay is currently more expensive than microscopy.

The review included studies that were primarily performed in reference laboratories as a part of research investigations; the studies also took place predominantly in countries with a high TB burden. The authors note that this could contribute to the evidence base of future reviews on Xpert in multiple healthcare settings, including situations when the test is performed at the point of care.

Every year, millions of people develop TB. Moreover, despite being curable, TB kills 3 people every minute. In a related news release, Dr. Steingart said, "This study represents the most comprehensive review on the diagnostic accuracy of Xpert to date and may help countries make decisions about scaling up its use for management of TB and drug-resistant TB."

J. Walton Tomford, MD, from the Cleveland Clinic in Ohio, tells Medscape Medical News that the review was long and comprehensive and should be reassuring for resource-poor countries that have the most cases of TB.

He explained that the assay's technology is "relatively simple and can be done by people without a lot of training. You do have to have a reliable electrical supply, however."

The authors have disclosed no relevant financial relationships.

Cochrane Library. Published online January 31, 2013. Full text