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23-08-2011 | General practice | Article

Serologic tests “not recommended” for TB diagnosis

Abstract

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MedWire News: Commercially available serologic tests for tuberculosis often have poor sensitivity and specificity and are not cost-effective, according to a report published in the journal PLoS Medicine.

The systematic review and meta-analysis showed that tests such as enzyme-linked immunosorbent assays (ELISA) and immunochromatographic assays that detect antibodies to TB are "inconsistent and imprecise," say Madhukar Pai (McGill University, Montreal, Canada) and colleagues.

The researchers explain that serologic tests for TB are widely used in developing countries, despite the current International Standards for TB Care discouraging the use of such tests in routine practice and no international guideline recommending their use.

As part of a World Health Organization (WHO) policy process, Pai and team updated a 2006 review of the diagnostic accuracy of commercial serologic tests for pulmonary and extrapulmonary TB to include reports published until June 2010. They paid particular attention to the relevance of these tests in low- and middle-income countries.

For pulmonary TB, they identified 67 studies (48% from low- and middle-income countries), with 5147 participants, which reported the sensitivity and specificity of serologic diagnostic tests (81% ELISA, 18% immunochromatographic, 1% kaolin precipitation test).

Pai and team report that sensitivity estimates for the tests varied among the studies from 0% to 100%, while specificity varied from 31% to 100%.

Only one test, the anda-TB immunoglobulin G (IgG) ELISA, was evaluated in enough studies to perform a meta-analysis. The results showed a pooled sensitivity of 76% in patients with positive smear microscopy (reference test) and 59% in smear-negative patients; pooled specificities were 92% and 91%, respectively.

When the researchers compared immunochromatographic tests with ELISA, they found that the immunochromatographic tests had lower pooled sensitivity (53% vs 60%, respectively) but comparable pooled specificity (98% vs 98%, respectively).

Pai and team also identified 25 studies (40% from low- and middle-income countries) that evaluated serologic test performance in 1809 patients with extrapulmonary TB. In these studies, sensitivity ranged from 0% to 100%, while specificity ranged from 59% to 100%.

Overall, the researchers graded the quality of evidence as very low for studies of pulmonary and extrapulmonary TB. Indeed, only 19 (28%) pulmonary TB studies and one (4%) extrapulmonary TB study included a representative patient population, while just 34 (51%) and four (16%) studies, respectively, reported blinding of the serologic test result.

Using the results from this study, Pai and co-authors conducted a cost-effectiveness study of serologic testing for active TB in India, also published in PLoS Medicine.

"In comparison with sputum microscopy, serologic testing resulted in fewer disability-adjusted life years averted and more false-positive diagnoses and secondary infections, while increasing costs to the Indian TB control sector approximately 4-fold," they write.

Together, the results of the two studies were used to guide a WHO policy statement on commercial serodiagnostic tests for diagnosis of TB.

The policy states that: "There is no evidence that existing commercial serologic assays [for TB] improve patient-important outcomes, and high proportions of false-positive and false-negative results adversely impact patient safety."

"It is therefore recommended that these tests should not be used in individuals suspected of active pulmonary or extra-pulmonary TB, irrespective of their HIV status.'

By Laura Dean

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