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GeneXpert test gives false outcomes of major drug-resistant TB cases

New Delhi: In a setback that could thwart the ambitious plans of Indian government and other health agencies to control the occurrence and spread of tuberculosis (TB), the recently introduced GeneXpert test, in a research, has been found to miss out significant number of cases of drug resistant TB.

Dr Sarman Singh

In a research carried at the All India Institute of Medical Sciences (AIIMS) here, it has been established that while the GeneXpert test still gives nearly accurate result in the diagnosis of tuberculosis, it failed to give satisfactory results in testing the drug sensitivity of the disease.

According to the study, one out of every three sputum samples put to test using this technology gave false sensitivity to TB drug (Rifampicin) when they were originally drug-resistant. The findings have been published in the Journal of Clinical Microbiology.

Dr Sarman Singh, professor and head of the clinical microbiology and molecular medicine division at AIIMS, told India Medical Times, “There are primarily two uses of GeneXpert test. The first one is related to the diagnosis of the disease and the second one is to test the drug susceptibility. Drug susceptibility is tested to determine whether the bacteria would respond to a particular drug or not and which line of treatment would be required.”

Dr Singh said, “The main findings of our research was that while the GeneXpert assay is effective for tuberculosis diagnosis, it has given wrong assessment in one third of the cases where the samples were put to test using this technology, giving false sensitivity to TB drug (Rifampicin) when they were originally drug-resistant.”

The researchers did the comparative analysis of RIF mono-resistance Line Probe Assay (LPA) results with the results of Xpert MTB/RIF. It was found that only 64.4 per cent RIF mono-resistant TB cases were correctly diagnosed by Xpert. The remaining 35.6 per cent cases were detected falsely RIF susceptible.

“So while we still believe that GeneXpert test can be relied upon for the diagnosis purpose, it cannot be solely relied for the identification of rifampin-resistant strains. Here a patient diagnosed with TB should go for further checkups to determine whether he is resistant to rifampin,” Dr Singh said.

This deadly form of the airborne disease remains a major public health problem worldwide, accounting for millions of deaths every year. The emergence and spread of multidrug resistant (MDR) and extensively drug-resistant (XDR) Mycobacterium tuberculosis complex (MTBC) strains pose significant challenges to disease control.

In order to overcome conventional methods’ low sensitivity and diagnostic delays, new molecular diagnostic tests were notified by the World Health Organization — Gene Xpert and Line Probe Assay besides two other tests, LED Microscope and Liquid Culture. UNITAID and the WHO are rolling out GeneXpert on massive scale that shortens the time to diagnose drug-resistant strains of TB from weeks to only a few hours.

But the latest findings could be a big blow at both individual and national level in Indian context. Dr Singh said, “At individual level, though the test would diagnose if a person has been affected by TB but can miss out on the seriousness of it. In the scenario of false results, the patient might be given first line of treatment whereas he could be resistant and may not respond to the drugs. So, caution and other tests are advised.”

“At national level, the ambitious plan of the government to install more than hundred machines at various testing centres might not prove to be fruitful as the test misses out on almost one third of the cases. The results would present low rifampicin resistance cases that could significantly affect country level data by recording underrated number of resistance cases. The cases which are actually resistant would be missed. Complacency by government and health workers would be inevitable in these circumstances,” he said.

Dr Singh further said, “The manufacturers of the machine should design the models by keeping in mind the country specific cases. For example, the test was a huge success in Africa but what we found here during sequencing was that the mutations were not picked by the cartridges in the machine. This should be considered from now on that the same cartridges may not work for every country.”

“Every country is installing the machines without much attention to its local use and impact because WHO has approved and advised it. It is time that every country should evaluate the samples carefully and demand the manufacturers to adopt country specific approach,” he opined.

The device could be a great way forward in the diagnosis and treatment of TB, but till the time the loopholes related to the cartridges and improved versions of the machine gets fixed, multiple tests to determine drug resistance should be adopted. A country dealing with one of the highest cases of MDR-TB cannot afford to miss out even a single case, which would only hamper the national programme of TB control.

by Vidhi Rathee

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