Tuberculosis (TB) Skin Test

What is a tuberculosis skin test?

The tuberculosis skin test is a screening test that can show if your child has 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, your child may need other tests to diagnose TB.

Why is this test done?

TB is spread through the air from person to person. TB usually causes lung disease, although it can affect almost any part of the body. 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.

Your child should get tested if he:

• Has been in close contact with someone known or thought to have TB
• Has HIV infection or a long-term health condition that weakens the immune system and puts your child at high risk for TB. The immune system is the body’s defense against infection.
• Has symptoms of TB (fever, night sweats, cough, and weight loss)
• Has been in close contact with people where TB is more common, such as a homeless shelter, migrant farm camp, prison or jail, or some nursing homes
• Has traveled to a country where TB is common or has 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)
• Uses illegal drugs

How do I prepare my child for this test?

Usually no preparation is needed for this test. Your child will need to come back to have his skin test checked in 48 to 72 hours.

Tell your healthcare provider if your child has 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 your child has TB even when he does not.

Talk to your healthcare provider if you have any questions about the test.

How is the test done?

For this test, your child's healthcare provider uses a needle to inject a small amount of fluid into the top layer of your child's skin. The fluid contains protein from the tuberculosis bacteria.

Your child needs to come back to the office in 2 to 3 days to have his skin checked for a reaction.

What does the test result mean?

If after 2 or 3 days your child has a red, raised, firm area around the test site, then he has probably been infected with TB. If his 3-day check was normal, but redness appears after the first 3 days, tell your child’s provider right away. Sometimes it takes longer than 3 days for the skin to show a reaction. Your child’s provider will decide if the redness is from a TB infection or some other cause.

A positive result does not always mean your child has an active TB infection. It may mean that your child was exposed and infected with TB in the past, but his body’s immune system is keeping the bacteria under control. Your child's skin could also show a reaction even though he is not infected. This is called a false positive result.

If your child is not infected, there will be no reaction in the area of the injection. This is a negative result. Your child may also have no reaction if he was infected in the last 6 weeks. Your child's skin could also show no reaction even though he is 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 child’s medical history, physical exam, and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about the result and ask questions such as:

• If your child needs more tests
• What kind of treatment your child might need
• What lifestyle, diet, or other changes your child might need to make

If the test is positive, close family members should be tested.