Tuberculosis (TB) Skin Test

What is a tuberculosis skin test?

The tuberculosis skin test is a test that can show if your child has been infected with tuberculosis (TB) bacteria. The test is called the Mantoux tuberculin skin test (TST).

Why is this test done?

Tuberculosis usually causes lung disease, although it can affect almost any part of the body. TB infections are not common in the US. 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.

Most people who are infected with TB bacteria have a form of infection called latent TB. They usually do not develop active TB, which means that they have no signs or symptoms of illness. Their body’s defenses control the infection by building a wall around the bacteria. This doesn’t get rid of the bacteria, but the bacteria stop growing. If your child has latent TB and becomes weak or ill, or your child’s immune system stops working well, the TB bacteria can become active. The bacteria can then multiply and your child may get sick.

If the test finds that your child has TB bacteria, you can be treated with medicine that will keep the infection from becoming active and spreading to others.

Most children in the U.S. do not need to be routinely tested. However, children at high risk for TB should have skin tests. A child’s risk may be high if:

• The child has been in close contact with people who have known or suspected TB infections.
• The child has traveled to a country where TB is common or has had a lot of contact with a person from such a country.

These children should have periodic TB skin tests:

• children who have chronic conditions such as cancer, diabetes, kidney failure, malnutrition, or certain types of immune system problems if they may have been exposed to TB
• children with HIV or in contact with persons with HIV
• children who were imprisoned
• children in contact with homeless people; people who live in nursing homes, dorms, or other facilities; or migrant farm workers

How do I prepare my child for this test?

Usually no preparation is needed for this test. Just be sure you can bring your child back to have his or her skin checked in 48 to 72 hours. Otherwise the test will need to be repeated.

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. It is like getting a shot.

Your healthcare provider will want your child to come back to the office in 2 to 3 days to check the area that was injected.

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 your child has probably been infected with TB. The lump will be measured to see if it is large enough to be seen as a positive result. A positive result does not necessarily mean your child has an active TB infection. Most people with positive tuberculin tests do not have active TB. A positive test without active disease simply means that sometime in the past your child was exposed and infected with TB, but their body’s immune system is keeping the bacteria under control.

If your child is not infected, there may be no reaction in the area of the injection. This is a negative result. Your child may also have no reaction if he or she was just recently infected (in the last 6 weeks).

Sometimes it takes longer than 3 days for the skin to show a reaction. Tell your provider if any redness appears after the first 3 days. Your provider will decide if the redness is from a TB infection or some other cause.

Sometimes the test may give a false-positive or false-negative result. This means that the skin may show a reaction even though your child is not infected (false positive) or there is no reaction even though your child is infected (false negative).

This test does not tell whether the infection is latent or active. Other tests, such as a chest X-ray and a sample of sputum, are needed to see if the infection is an active disease.

Many people born outside of the US have been vaccinated against TB with the Bacillus Calmette-Guérin (BCG) vaccine. This vaccine is not routinely given in the U.S. It may cause a positive reaction to the TB skin test, which may complicate decisions about prescribing treatment. The BCG vaccine may cause the skin to have a reaction after a TB skin test, so that it looks like your child has TB even when he or she does not. If your child has had the BCG vaccine, be sure to tell your healthcare provider.

What if the test result is not normal?

If the test is positive, close family members should also have the test.

Test results are only one part of a larger picture that takes into account your child’s medical history 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
• when your child needs to be tested again
• what lifestyle, diet, or other changes your child might need to make

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