Much-hyped new tuberculosis test gives inaccurate results

NEW DELHI: In a major setback to improving tuberculosis (TB) diagnosis and treatment in the country, researchers have found that the new gene Xpert gene test being promoted by government and top health agencies of the world does not give accurate results. In fact, one out of every three sputum sample put to test using this technology gave false sensitivity to TB drug (Rifampicin) in study carried at the All India Institute of Medical Sciences (AIIMS) when they were originally drug-resistant.

Dr Sarman Singh, professor and head of the clinical microbiology and molecular medicine division at AIIMS, said GeneXpert has been a revolutionary diagnostic method in Africa but in India it can miss as many as one-third of Rifampicin-resistance cases. "The Indian strains have a peculiar gene sequence which is not recognized by the probes GeneXpert has. Hence, if such systems are used routinely, this would give a false impression that India has very low rifampicin resistance thus making the programme mangers complacent," Dr Singh added.

There are four tests approved by WHO: LED Microscope, Liquid Culture and two molecular tests - Gene Xpert and Line Probe Assay. Gene Xpert, one of the advanced tests for TB diagnosis, is available at AIIMS for a patient suffering from drug resistant TB but it is not available in all hospitals due to its high cost. But central TB division of Ministry of Health, Government of India is installing GeneExpert in all the referral TB laboratories.

Dr Singh said the national TB control program managers must evaluate the performance of Xpert MTB/RIF test before rolling it out in the Drug resistant-TB control program in view of the findings, which has been published in latest issue of the Journal of Clinical Microbiology. The AIIMS study, researchers said, was done in a double-blinded manner. "After getting the RIF mono-resistance Line Probe Assay (LPA) results, one of us asked the persons in charge for Xpert MTB/RIF to run these samples in Xpert using the newer version of cartridges as per manufacturer instruction. Comparative analysis showed only 64.4% RIF mono-resistant TB cases were correctly diagnosed by Xpert. The remaining 35.6% were detected falsely RIF susceptible," said an AIIMS researcher.

In an earlier report published in PLOS Medicine journal, researchers pointed out that Xpert MTB/RIF has a number of limitations including limited shelf-life of the diagnostic cartridges, operating temperature and humidity restrictions, requirement for electricity supply, the need for annual servicing and calibration of each machine.

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Much-hyped new tuberculosis test gives inaccurate results