July 27, 2011

New TB-Drug Resistance Test

(Ivanhoe Newswire) -- Two research studies suggest that a new automated DNA test for tuberculosis (Xpert MTB/RIF), which can detect TB within 2 hours and has been endorsed by the World Health Organization, can significantly increase TB detection rate compared to other tests, particularly in HIV positive patients who have a high risk of being infected with TB, including multidrug resistant TB.

In the first study, led by Stephen Lawn from the Desmond Tutu HIV Centre at the University of Cape Town in South Africa, the authors collected sputum from HIV-infected adults with no current TB diagnosis who were enrolling at an HIV treatment clinic in a South African township. The authors then compared the diagnostic accuracy of Xpert MTB/RIF with several other tests, including liquid culture (the reference test).

Nearly a fifth of the patients had culture-positive TB and Xpert MTB/RIF identified three-quarters of these patients. Furthermore, the new test had a low false-positive rate and was able to detect all cases of smear-positive, culture-positive TB but only 43.4% of smear-negative, culture-positive cases from a single sputum sample.
The new test also correctly identified rifampicin resistance, a marker for multidrug resistant TB, in all four patients who had this form of TB, but incorrectly identified resistance in three patients with drug-sensitive TB.

"In this population of individuals at high risk of TB, intensive screening using the Xpert MTB/RIF assay increased case detection by 45% compared with smear microscopy, strongly supporting replacement of microscopy for this indication," the authors were quoted as saying. "However, despite the ability of the assay to rapidly detect rifampicin-resistant disease, the specificity for drug resistant TB was sub-optimal."

SOURCE: PLoS Medicine, published online July 26, 2011