Diagnosing pulmonary TB – TB that affects the lungs – can be difficult, and several tests are usually needed.
You may have 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 the most effective treatment for you.
Several tests can be used to confirm suspected extrapulmonary TB, which is TB outside the lungs.
These tests include:
- a CT scan, MRI scan or ultrasound scan of the affected part of the body
- an examination of the inside of your body using a long, thin, flexible tube with a light and camera at one end (endoscopy) – the endoscope can be inserted through a natural opening, such as your mouth, or through a small cut made in your skin (laparoscopy) if there's a need to check other parts of your 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, where a small sample of cerebrospinal fluid (CSF) is taken from the base of your spine. CSF is fluid that surrounds the brain.
The sample can be checked to see whether TB has infected your brain and spinal cord (central nervous system).
Testing for latent TB
In some circumstances, you may need to have a test to check for latent TB – where you've been infected with TB bacteria, but do not have any symptoms.
For example, you may need to have a test if you've been in close contact with someone known to have active TB disease involving the lungs, or if you've recently spent time in a country where TB levels are high.
If you've just moved to the UK from a country where TB is common, you should be given information and advice about the need for testing. Your GP may suggest having a test when you register as a patient.
The Mantoux test is a widely used test for latent TB. It involves injecting a small amount of 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 small, 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 active TB disease.
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've 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 blood test for TB that's 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 – the Mantoux test may not be reliable in these cases
- as part of your TB screening if you've 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're about to have treatment that will suppress your immune system
- if you're a healthcare worker