Several tests are used to diagnose tuberculosis (TB). The test you have will depend on the type of TB that is suspected. Your GP may refer you to a TB specialist if they think that you have TB.
Pulmonary TB
A diagnosis of pulmonary TB (TB that affects the lungs) can usually be confirmed with a chest X-ray. This uses high-energy radiation to create an image of your lungs. If you have a TB infection, changes to the appearance of your lungs, such as scarring, should be visible on the X-ray.
Samples of mucus and phlegm will also be taken and checked under a microscope for the presence of TB bacteria.
Extrapulmonary TB
If you have suspected extrapulmonary TB (TB that occurs outside the lungs), several tests can be used to confirm a diagnosis. These include:
- computerised tomography (CT) scan - a series of X-rays of your body is taken at slightly different angles and a computer puts the images together to create a detailed picture of the inside of your body
- magnetic resonance imaging (MRI) scan - a strong magnetic field and radio waves are used to produce detailed images of the inside of your body
- ultrasound scan - high-frequency sound waves create an image of part of the inside of your body
- blood test
- urine test
- biopsy - a small tissue sample is taken from the affected site and tested for the presence of disease
You may also have a lumbar puncture. This involves taking a small sample of cerebrospinal fluid from the base of your spine. Cerebrospinal fluid is a clear fluid that surrounds and supports the brain. The cerebrospinal fluid will be checked to see whether TB has infected your central nervous system.
Screening for latent TB
In some circumstances, you may need to be screened to check for a latent TB infection. This is when you have been infected with the TB bacteria but you do not have any symptoms.
For example, you may need to be screened if you have been in close contact with someone who is known to have an active TB infection, 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.
If you need to be screened for TB, you may be referred to the Health Protection Agency (HPA). The HPA helps protect public health by providing support and advice to organisations such as the NHS. They have offices across England and may be your point of contact if you are involved in an outbreak of TB or need to be screened. See the HPA website for more information about how they are involved in cases of TB.
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.
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 to have 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 some time to develop, you may need to be screened again within a year.
If you have had the Bacillus Calmette-Guérin (BCG) vaccination, you may have a mild skin reaction to the Mantoux test. This does not mean that you have latent TB, but that your immune system (the body’s natural defence against infection and illness) recognises the 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
- as part of your TB screening if you have just moved to the UK from a country where TB is common
- if you are about to have treatment that will suppress your immune system, such as a type of medicine called tumour necrosis factor blockers
- if you are a healthcare worker
