Cutting Edge: New TB test kit can detect latent infection

With India chasing the target of eliminating tuberculosis by 2025, five years ahead of the global sustainable development goal, Union Health Minister Mansukh Mandaviya recently announced a “Made in India” TB skin test to be used in the disease control mission.

The test, which will be manufactured and marketed as Cy-TB by the Serum Institute of India, is a slightly tweaked version of a century-old test where a protein extract of tuberculosis bacteria was injected under the skin to test for reaction to the infection.

Why is the test necessary? Because it can help detect latent tuberculosis infections. At present, the national TB programme focuses solely on detecting and treating those with an active infection, meaning those who have already started showing symptoms. There are two methods of detecting an active infection – microscopy, where a lab technician literally checks the patient sample under the microscope, or the nucleic acid amplification test, where the genetic material of the pathogen is amplified and its segments are automatically matched by the machine to give a positive result (just like an RT-PCR test for COVID-19). These tests, however, use a patient’s sputum sample and cannot detect a latent infection.

It is estimated that almost one-fourth of the world’s population carries the TB bacteria in the latent form, but some of them may go on to develop an active infection, especially if their immune system gets compromised by other illnesses or medication. Although people with latent TB cannot spread the infection, they act as a reservoir for the pathogen to persist in humans.

For detecting the latent TB cases, either a blood test or a skin test can be performed. The problem with the hundred-year-old skin test, also known as Mantoux test, is that it also shows positive for those who have received the BCG vaccine for tuberculosis. BCG vaccine is a part of the routine immunisation for children in India, making the test useless.

The new test, instead of using a protein extract of the entire bacterium, measures the body’s immune response to two of the antigens or infective parts of tuberculosis — EAST-6 and CFP10.

To perform the test, the compound is injected just under the skin on the arm which forms a bump. This is then measured two to three days later to see whether the person has the infection. The bump grows larger if they are carriers of the pathogen.

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The test was developed by the Statens Serum Institute, Denmark with which SII has a partnership. It is relatively cheaper than the blood tests for latent TB and does not require laboratory facilities. The skin test can be used to screen for TB in the community.

A clinical trial has found that it is as safe as the Mantoux test but has better sensitivity in the general population as well as household contacts of a TB patient and those at a high risk of contracting the disease. The test will be available for adults while the company will be conducting a trial in those below the age of 18 years, according to the subject expert committee under India’s apex drug regulator.

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