TB - Diagnosing tuberculosis

Several tests are used to diagnose tuberculosis (TB), depending on the type of TB suspected.

Your GP may refer you to a TB specialist for testing and treatment if they think you have TB.

Pulmonary TB

A diagnosis of pulmonary TB (TB that affects the lungs) can be difficult and several tests are usually needed.

This will include a chest X-ray to look for changes in the appearance of your lungs that are suggestive of TB. Samples of phlegm will also often be taken and checked for the presence of TB bacteria.

These tests are important in helping to decide on the most effective treatment for you.

Extrapulmonary TB

If you have suspected extrapulmonary TB (TB that occurs outside the lungs), several tests can be used to confirm a diagnosis. These may include:

a computerised tomography (CT) scan, magnetic resonance imaging (MRI) scan, or ultrasound scan of the affected part of the body
urine and blood tests
a biopsy – a small sample of tissue or fluid is taken from the affected area and tested for TB bacteria

You may also have a lumbar puncture. This involves taking a small sample of cerebrospinal fluid (CSF) from the base of your spine. CSF is fluid that surrounds the brain. It can be checked to see whether TB has infected your central nervous system (brain and spinal cord).

Screening for latent TB

In some circumstances, you may need to be tested to check for latent TB (when you have been infected with TB bacteria but do not have any symptoms).

For example, you may need to be screened if you have been in close contact with someone known to have an active TB infection (an infection that causes symptoms), or if you have recently spent time in a country where TB levels are high.

If you have just moved to the UK from a country where TB is common, you may be screened when you arrive or your GP may suggest screening when you register as a patient.

Mantoux test

The Mantoux test is a widely used test for latent TB. It involves injecting a substance called PPD tuberculin into the skin of your forearm. It's also called the tuberculin skin test (TST).

If you have a latent TB infection, your skin will be sensitive to PPD tuberculin and a hard red bump will develop at the site of the injection, usually within 48 to 72 hours of having the test. If you have a very strong skin reaction, you may need a chest X-ray to confirm whether you have an active TB infection.

If you do not have a latent infection, your skin will not react to the Mantoux test. However, as TB can take a long time to develop, you may need to be screened again at a later stage.

If you have had the BCG vaccination, you may have a mild skin reaction to the Mantoux test. This does not necessarily mean you have latent TB.

Interferon gamma release assay (IGRA)

The interferon gamma release assay (IGRA) is a newer type of blood test for TB that is becoming more widely available.

The IGRA may be used to help diagnose latent TB:

if you have a positive Mantoux test
if you previously had the BCG vaccination (as the Mantoux test may not be reliable in these cases)
as part of your TB screening if you have just moved to the UK from a country where TB is common
as part of a health check when you register with a GP
if you are about to have treatment that will suppress your immune system
if you are a healthcare worker