Diagnosing TB - Which test to use, molecular tests

To diagnose TB it is necessary to carry out one of a number of different TB tests.

Diagnosing TB, what do the tests look for?

The tests are looking to see if the person has TB bacteria in their body.  If a person has TB bacteria in their body they may have latent TB infection or TB disease. Latent TB infection means that they do not have any symptoms for TB and they cannot pass TB on to other people.

But the person with TB bacteria may also have symptoms of TB. This means that they have TB disease. They need to be treated for TB disease and they can pass TB on to other people.

Diagnosing TB, what tests can be used?

The skin test is widely used for diagnosing TB in countries with low rates of TB. The problem with using it in countries with  high rates of TB is that the majority of people may have latent TB.

Sputum microscopy is often the first test to be used to diagnose TB in a country with a high rate of TB infection. Sputum is a thick fluid that is produced in the airways leading to the lungs. A sample of sputum is usually collected by the person coughing.

Sputum smear microscopy is inexpensive and simple, and people can be trained to do it relatively quickly and easily. But its accuracy is not very good.1Siddiqi, Kamran, "Clinical diagnosis of smear-negative pulmonary tuberuclosis in low-income countries: the current evidence", The Lancet Infectious Diseases, Vol 3, May 2003,288 www.thelancet.com/

The GeneXpert is one of the new molecular tests that can diagnose TB

The GeneXpert is one of the new molecular tests that can diagnose TB

So in some countries sputum smear microscopy is being phased out and is being replaced by molecular TB tests. Molecular tests can also provide information about drug resistance.

There is more information about the complete range of TB tests that are used to diagnose TB.

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This page was last updated in June 2021.
Author Annabel Kanabus

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