What does the Tuberculin Skin Test (TST) measure?
The TST (or Mantoux test) indicates whether a person has been infected with Mycobacterium tuberculosis (the bacteria causing TB) by measuring if the body’s immune response to TB has been stimulated. A TST may be used to diagnose latent TB infection. It is not used to test for TB disease.
How is it performed?
A TST involves a small injection of tuberculin (or purified protein derivative – PPD) into the skin, usually on the forearm. After 48-72 hours the injection site is assessed for any reaction (lump). If a lump develops it will disappear within days without any treatment.
What is latent TB infection?
Latent TB infection is when a person has the TB bacteria in their body but the immune system is stopping the infection from causing disease. Latent TB infection causes no symptoms and cannot be passed on to other people.
There is a small risk that latent TB infection can progress to TB disease. The risk is increased in young children and old people, as well as people with weak immune systems. TB disease develops if the TB bacteria start to multiply and cause symptoms, such as a fever, unexpected weight loss, tiredness or a cough that won’t go away. Latent TB infection can be treated with special antibiotics to prevent it from progressing to TB disease in the future.
Other tests for latent TB infection
The interferon gamma release assay (IGRA) is another test for latent TB infection. It is a blood test and also measures the body’s immune response to TB bacteria.
Your nurse or doctor will explain the advantages and disadvantages of the different tests and advise which is more suitable for your situation. In some situations both TST and IGRA testing may be required.
Before you have a TST
It is important to let the nurse or doctor know if you:
- Have any immune-suppressing illness such as HIV, lymphoma, or kidney disease
- Take medication that affects your immune system such as steroids (e.g. prednisone), cyclosporine or chemotherapy (cancer drugs )
- Have had a fever or infection in the past month, such as influenza, measles, or pneumonia
- Have received any vaccines in the past month
- Have had TB disease in the past, had contact with someone with TB, have received the BCG vaccine, or have travelled or lived overseas
These conditions may affect the test result.
Care of the TST injection site
- Do not scratch the site
- Do not cover the site with any dressing, cream or ointment
- If any blisters occur do not break them
- You may continue all normal activities including work, sports and showering.
After you have a TST
After the TST you must return to the clinic 48-72 hours later so that any reaction can be assessed and recorded. Some people who have a negative TST may need a second TST, while some people with a positive TST may require an IGRA (blood test). Your nurse or doctor will explain why further tests may be needed.
Contact your local TB service for more information