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.

Several tests are usually needed, including a chest X-ray. This uses 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.

Other tests may also be necessary to confirm a diagnosis. For example, samples of mucus and phlegm will also be taken and checked under a microscope for the presence of TB bacteria.

These tests are important in helping decide what will be the most effective treatment.

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:

  • a 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
  • a magnetic resonance imaging (MRI) scan – a strong magnetic field and radio waves are used to produce detailed images of the inside of your body
  • an ultrasound scan – high-frequency sound waves create an image of part of the inside of your body
  • blood tests 
  • urine tests
  • a 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 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 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.

If you need to be screened for TB, you may be referred to Public Health England (PHE) or a local TB clinic. PHE helps protect public health by providing support and advice to organisations such as the NHS.

PHE or your local TB clinic will be your point of contact if you are involved in an outbreak of TB or need to be screened. See the PHE website or your local hospital 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 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 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 you have latent TB, but that your immune system (the body’s natural defence against infection and illness) recognises 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
  • 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 

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