India's fight against tuberculosis seems to have hit a roadblock with a study raising serious doubts on the accuracy of a diagnostic test being promoted by the government and top health agencies of the world.
GeneXpert test, which is used to detect multidrug resistant TB in two hours, gives false sensitivity to drug Rifampicin in one out of every three sputum samples, according to a research by the All India Institute of Medical Sciences (AIIMS).
The researchers have said that Indian strains have a peculiar gene sequence which is not recognized by the GeneXpert method and therefore, routine use of the technology can give wrong information on rifampicin resistance.
The study has been published in latest issue of the Journal of Clinical Microbiology. Mumbai presently has six machines in state-run hospitals and more than six in private sector to carry out the new test for TB as well as for resistance to the drug.
Dr Yatin Dholakia, secretary of Maharashtra state anti-TB association said: "Gene Xpert has certain limitations as it is a process of real-time (PCR) based test resulting in low sensitivity. Clinical history, examination, past history of TB and drug treatment needs to be evaluated along with this test. There are other tests like LPA and culture test."
The civic body's health department is however not worried with the AIIMS study. Dr Mini Khetrapal, Mumbai tuberculosis officer, said: "We only subject those TB patients to the test who we suspect are multi-drug resistant. New tuberculosis suspect patients are not sent for geneXpert. Therefore there is question of missing patients."
There are four tests approved by WHO: LED Microscope, Liquid Culture and two molecular tests - GeneXpert and Line Probe Assay. The central TB division of the union ministry of health is installing Genexpert machines in all the referral TB laboratories.
"We are following the guidelines set by the central TB division," said Dr Khetrapal.
Mumbai TB figures
There are at present 146 extensively drug resistant (XDR) TB cases in Mumbai and 4,697 progressive MDR TB patients
TB cases in 2012: 30,828; 2013: 31,789; so far in 2014: 5,213
MDR TB cases in 2012: 1,679; 2013: 2,394; so far in 2014: 453
TB cases notified by labs to BMC so far: 6,961; by hospitals: 2,685
What is PCR?
Sputum sample of the patient is fed to the machine and with the biochemical reactions, it detects whether the sample contains the TB bacterium. The machine looks for the DNA specific to the TB bacterium. If there are TB bacteria in the sample, the machine will detect their DNA and automatically multiply it. This technique is called PCR (polymerase chain reaction), and allows the machine to also look at the structure of the genes.
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