What is Tuberculin Skin Test

Tuberculin skin test is used to check whether a person has been exposed to the bacteria that causes tuberculosis (TB) (Mycobacterium tuberculosis). Tuberculosis is a bacterial infection that usually affects the lungs but can affect other parts of the body. You may have a tuberculin skin test if:

• You have possible symptoms of TB, such as:
  • Coughing up blood, mucus from the lungs (sputum), or both.
  • A cough that lasts three weeks or longer.
  • Chest pain, or pain while breathing or coughing.
  • Unexplained weight loss.
  • Fatigue and weakness.
  • Fever, sweating, and chills.
  • Loss of appetite.
• You are at high risk for getting TB. You may be at high risk if you:
  • Inject illegal drugs or share needles.
  • Have HIV or other diseases that affect the disease-fighting (immune) system.
  • Work in a health care facility.
  • Live in a high-risk community, such as a homeless shelter, nursing home, or correctional facility.
  • Have had contact with someone who has TB.
  • Are from or have traveled to a country where TB is common.

If you are at high risk, you may need to have regular TB screenings. TB screening may be required when starting a new job, such as becoming a health care worker or a teacher. Colleges or universities may require TB screening for new students.

What is being tested?

This test checks for the presence of TB antibodies in the body. Antibodies are a type of cell that is part of the body’s immune system. After you get an infection, your body makes antibodies that stay in your body after you recover and protect you from getting the same infection again.

Tell a health care provider about:

• Any allergies you have.
• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders you have.
• Any surgeries you have had.
• Any medical conditions you have.
• Whether you are pregnant or may be pregnant.

What happens during the test?

Your health care provider will inject a solution called PPD (purified protein derivative) under the first layer of skin on your arm. This causes a small, blister-like bump to form over the area temporarily. PPD is made from the bacteria that causes TB. PPD causes your immune system to react, but it does not get you sick with TB.

You may feel mild stinging as this happens. Afterward, the area may itch or burn.

How are the results reported?

To get your test results, you will need to see your health care provider again within 2–3 days after you received the injection. It is important to follow your health care provider’s instructions about when to be seen again. If you are not seen within 2-3 days, you may need to have the test repeated. At your follow-up visit, your health care provider will measure the area where the PPD was injected to see if the bump has gotten larger due to swelling. Your results will be reported as positive or negative:

• If the bump has disappeared or is small, your test result is negative. Negative means that you do not have the antibodies.
• If the bump is large, your test result is positive. Positive means that you have the antibodies. Swelling is caused by the antibodies reacting with the PPD. The skin may also turn red around the bump.

A false-positive result can occur. A false positive is incorrect because it means that a condition is present when it is not.

A false-negative result can occur. A false negative is incorrect because it means that a condition is not present when it is. False negatives are rare and are more likely to occur in older people and in people who have weakened immune systems.

What do the results mean?

A negative result means that it is unlikely that you have TB or that you have been exposed to TB bacteria. This test may be repeated, or you may have a blood test to check for TB. This is because your body may not react to the tuberculin skin test until several weeks after exposure to TB bacteria.

A positive result means that you have been exposed to TB, and you may need more tests to determine if you have:

• Active TB, also called TB disease. This means that you have TB symptoms and your infection can spread to others (you are contagious).
• Latent TB. This means that you do not have any symptoms of TB and you are not contagious. Latent TB can turn into active TB.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• The tuberculin skin test is used to check whether a person has been exposed to the bacteria that causes tuberculosis (TB).
• Your health care provider will inject a solution known as PPD (purified protein derivative) under the first layer of skin on your arm.
• After 2–3 days, your health care provider will measure the area where the PPD was injected to see if the bump has gotten larger due to swelling.
• Your results will be reported as positive or negative. A positive result means that you have been exposed to TB. A negative result means that it is unlikely that you have TB or that you have been exposed to TB bacteria.