Tuberculosis (TB) is a disease caused by bacteria (Mycobacterium tuberculosis). TB usually affects the lungs and sometimes spreads to other body tissues or organs. A person who becomes infected with TB may not have any symptoms at first, but may get sick years later. A person with a TB infection that is causing symptoms (called an active infection) can spread the disease to others.

TB bacterial cells contain substances called antigens. When a person becomes infected with TB, the immune system usually produces antibodies that destroy the bacteria.

A tuberculin skin test helps determine whether you have been infected by TB bacteria. During a tuberculin test, TB antigens (not entire TB bacterial cells) are injected into the skin. If a person has been previously exposed to TB bacteria, a reaction to the TB antigens will develop within 2 days, usually causing a firm red bump at the injection site.

TB skin tests use a preparation of TB antigens called purified protein derivative (PPD).

There are two main types of TB skin tests:

• Multiple tine test. This test uses a small "button" that has several short needles (tines) coated with TB antigens. The tines are pressed into the skin (usually on an arm), forcing the antigens into skin. A multiple tine test is used mainly for TB screening. Mono-vacc, Aplitest, and the tine test are names of multiple-tine tuberculin skin tests.
• Mantoux test (PPD skin test). This test uses a measured amount of TB antigens that are injected into the skin using a syringe and needle. A Mantoux test is more accurate than a multiple tine test. It is often used when symptoms or testing (such as a chest X-ray) indicate a person may have TB or to confirm the results of a multiple tine test.

A tuberculin skin test cannot determine how long a person has been infected with TB or whether the infection is active or inactive (dormant).

Some people do not react to a tuberculin skin test, even if they have tuberculosis. A weakened immune system (caused by conditions such as cancer or AIDS) does not always respond normally to the TB antigens. In these cases, other skin tests that detect an immune system reaction to very common organisms (such as tetanus and yeast) may be done. This is called anergy testing. If there is a skin reaction to the anergy tests, then the tuberculin skin test is probably accurate. However, if there is no reaction to the anergy tests, the person's immune system is likely to be too weakened or damaged to respond normally to the tuberculin skin test.

Tuberculin Skin Tests
Results

The width (diameter) of the firm area (not the reddened area) on the arm is measured 2 to 3 days after the injection to determine the test result. The person's risk factors and the type of test done are considered when interpreting the test result.

There are three levels of risk:

• High-risk group includes people with an HIV infection, those who have had close recent contact with a person who has an active tuberculosis (TB) infection, and those who have symptoms or other test results that indicate TB.
• Moderate-risk group includes people who have recently moved from or traveled in a country with a high rate of TB; those who use recreational drugs by injection (intravenous drug users); people who live in nursing homes; workers in hospitals, nursing homes, schools, and prisons; children less than 4 years of age; and homeless people.
• Low-risk group includes people who do not fall into any of the higher-risk categories.

A reaction usually remains visible for about 1 week.

• The size of bump that indicates a positive Mantoux test can vary among doctors.
• A positive tuberculin skin test does not mean you have a contagious infection that is causing symptoms (an active TB infection). The test cannot distinguish between an active and inactive infection, a recent or past infection, or an actual infection and a previous vaccination. Further tests (such as a chest X-ray, a sputum culture, or both) are usually done to determine whether you have an active TB infection.