Simple blood test can spot people with active tuberculosis

Scientists, including one of Indian origin, have identified a gene expression “signature” that could help accurately diagnose active tuberculosis (TB) using a simple blood test.

Globally, tuberculosis infects 9.6 million new patients each year and kills 1.5 million. Yet the disease remains difficult to diagnose. “One-third of the world’s population is currently infected with TB. Even if only 10 per cent of them get active TB, that’s still 3 per cent of the world’s population, 240 million people,” said senior author Purvesh Khatri, assistant professor at Stanford University in the US. Traditional diagnostic methods, such as the skin prick test and interferon assays, can not separate patients with active TB from those who are no longer sick or have merely been vaccinated against TB. These older diagnostics can miss a case of TB in patients with HIV. A common way to test for TB is to find disease-causing bacterium in sputum samples coughed up by pat-ients. However, sometimes it is hard for people to produce sputum on demand, said first author Tim Sweeney, research associate from Stanford. — PTI

The new test developed works on an ordinary blood sample and removes the need to collect sputum. It can signal a TB infection even if the individual also has HIV and won’t give a positive response if someone only has latent TB or has had a TB vaccine, researchers said. It also does not matter which strain of TB has infected a person, or even if it has evolved resistance to antibiotic drugs. The test works in both adults and children. The test is 86 per cent sensitive in children, and if the test comes up negative, it is right 99 per cent of the time. The requirements of the test are simple enough that it can potentially be done under relatively basic field conditions in rural and undeveloped areas of the world.

Any hospital should be able to perform the test. Villages without electricity could likely use ordinary blood samples and a solar-powered PCR machine, which multiplies strands of DNA, to accurately test people for active TB. Researchers identified three human genes whose expression changes in a consistent pattern, indicating the presence of an active tuberculosis infection.

The team validated the new three-gene test in a separate set of 1,400 human samples from 11 different data sets, confirming the diagnostic power of the test. The new test not only accurately distinguishes patients who have active tuberculosis, it could also be used to monitor patients to see if they are getting better and how well they are responding to different treatments. Thus, it can be used not only for diagnosis and to inform treatment, but also to study the effectiveness of different treatments.

The study was published in the journal Lancet Respiratory Medicine.