What is a tuberculosis skin test?
The tuberculosis skin test is a screening test that can show if you have been infected with tuberculosis (TB) bacteria. The test is called the Mantoux tuberculin skin test (TST). Another name is PPD (purified protein derivative). If the test is positive, you may need other tests to diagnose TB.
Why is this test done?
TB usually causes lung disease, although it can affect almost any part of the body. However, an active TB infection can be a very serious illness. The TB skin test can find most TB infections before the infection is serious enough to cause any problems. TB infections are no longer common in the US.
You should get tested for TB if you:
- Have been in close contact with someone known or thought to have TB
- Have HIV infection or a long-term health condition that weakens your immune system and puts you at higher risk for TB. The immune system is your bodyâ€™s defense against infection.
- Have symptoms of TB (fever, night sweats, cough, and weight loss)
- Live or work where TB is more common, such as a homeless shelter, migrant farm camp, prison or jail, or some nursing homes
- Have lived in or traveled to a country where TB is common or have had a lot of contact with a person from such a country (most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia)
- Use illegal drugs
How do I prepare for this test?
Usually no preparation is needed for this test. You will need to come back to have your skin test checked in 48 to 72 hours.
Tell your healthcare provider if you have had the BCG vaccine. This vaccine is used in countries where TB is common, but is rarely given in the US. It may cause the skin to react as if you have TB even when you do not.
Talk to your healthcare provider if you have any questions about the test.
How is the test done?
Your healthcare provider will use a needle to inject a small amount of fluid into the top layer of your skin. The fluid contains protein from the tuberculosis bacteria. You need to come back to the office in 2 to 3 days to have your skin checked for a reaction.
A 2-step test method is used for older people who may have a poorer immune response. If the first test is negative, the test is repeated in 7 to 10 days.
What does the test result mean?
If after 2 or 3 days you have a red, raised, firm area around the test site, then you have probably been infected with TB. If your 3-day check was normal, but redness appears after the first 3 days, tell your provider right away. Sometimes it takes longer than 3 days for the skin to show a reaction. Your provider will decide if the redness is from a TB infection or some other cause.
- A positive result does not always mean you have an active TB infection. It may mean that you were exposed and infected with TB in the past, but your bodyâ€™s immune system is keeping the bacteria under control.
- Your skin could also show a reaction even though you are not infected. This is called a false positive result.
- If you are not infected, there will be no reaction in the area of the injection. This is a negative result.
- You may also have no reaction if you were infected in the last 6 weeks. Your skin could also show no reaction even though you are infected. This is called a false negative result.
What if the test result is not normal?
Test results are only one part of a larger picture that takes into account your medical history, physical exam, current health, and risk for exposure. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about your result and ask questions such as:
- If you need more tests
- What kind of treatment you might need
- What lifestyle, diet, or other changes you might need to make
If the test is positive, close family members should be tested.
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This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
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