Xpert diagnosis of TB and its drug resistant strains

By Sally Robertson, B.Sc.Feb 14 2013

medwireNews: The use of a novel, diagnostic test called Xpert (Cepheid Inc, California, USA) is both sensitive and specific in detecting tuberculosis (TB) and rifampicin resistance, report US researchers.

The findings support World Health Organization (WHO) recommendations that Xpert should be used as an initial diagnostic test for detection of TB and rifampicin resistance in patients suspected to have multi-drug resistant (MDR)-TB or HIV-associated TB, they say.

"Since Xpert was endorsed by WHO, a large number of studies have been performed and country-level policy makers are making decisions about adoption and scale-up," explain Karen Steingart (University of Washington, Seattle) and colleagues from the Cochrane Infectious Diseases Group.

"This systematic review represents the most comprehensive review on the diagnostic accuracy of Xpert and provides evidence that may help countries make decisions about scaling up Xpert for programmatic management of TB and drug-resistant TB."

In a meta-analysis of 18 studies identified though a comprehensive database search, the team estimated the pooled sensitivity and specificity of Xpert using data from a total of 7816 individuals included in randomized controlled trials and cross-sectional and cohort studies. Most studies were conducted in low- and middle-income countries with a high burden of disease.

Writing in The Cochrane Library, the group explain that "until the development of Xpert, smear microscopy had been the sole method used for TB diagnosis in most laboratories in developing countries, where over 95% of TB-related deaths occur."

In the current study, the researchers found that when Xpert was used as an initial test replacing smear microscopy, it achieved a pooled sensitivity of 88% and a pooled specificity of 98%. And when used as an add-on test following a negative smear microscopy result, Xpert yielded a sensitivity of 67% and a specificity of 98%.

"Although a sensitivity of 67% may be considered low, it is precisely in smear-negative patients that improvements in diagnostic tests are needed and where any improved diagnostic test will have the biggest impact," explains the team. "Therefore Xpert may also be valuable as an add-on test following spear microscopy."

In subgroup analysis, the team found that the pooled sensitivity of Xpert was 98%for smear-positive, culture-positive TB and 68% for smear-negative, culture-positive TB. And in people living with HIV it was 80%, while in those without HIV infection it was 89%.

In addition, when used as an initial test replacing conventional drug susceptibility testing, Xpert yielded a sensitivity of 94% and specificity of 98% for detecting rifampicin resistance (a proxy for MDR-TB).

Commenting on the findings in a press statement, Lucic Ditiu, Executive Secretary of the Stop TB Partnership, said: "We welcome the Cochrane Review of Xpert, an innovation that represents a major milestone in our quest to achieve the goal of zero deaths from TB."

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