Tuberculosis (TB) is a disease that is spread through the air from one person to another. There are two kinds of tests that are used to determine if a person has been infected with TB bacteria: the tuberculin skin test and TB blood tests.
A positive TB skin test or TB blood test only tells that a person has been infected with TB bacteria. It does not tell whether the person has latent TB infection (LTBI) or has progressed to TB disease. Other tests, such as a chest x-ray and a sample of sputum, are needed to see whether the person has TB disease.
Tuberculin skin test: The TB skin test (also called the Mantoux tuberculin skin test) is performed by injecting a small amount of fluid (called tuberculin) into the skin in the lower part of the arm. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm. The health care worker will look for a raised, hard area or swelling, and if present, measure its size using a ruler. Redness by itself is not considered part of the reaction.
TB blood tests: TB blood tests (also called interferon-gamma release assays or IGRAs) measure how the immune system reacts to the bacteria that cause TB. An IGRA measures how strong a person’s immune system reacts to TB bacteria by testing the person’s blood in a laboratory.
Two IGRAs are approved by the U.S. Food and Drug Administration (FDA) and are available in the United States:
IGRAs are the preferred method of TB infection testing for the following:
There is no problem with repeated IGRAs.
Certain people should be tested for TB bacteria because they are more likely to get TB disease, including:
Many people born outside of the United States have been BCG-vaccinated.
People who have had a previous BCG vaccine may receive a TB skin test. In some people, BCG may cause a positive skin test when they are not infected with TB bacteria. If a TB skin test is positive, additional tests are needed.
IGRAs, unlike the TB skin tests, are not affected by prior BCG vaccination and are not expected to give a false-positive result in people who have received BCG.
The person’s health care provider should choose which TB test to use. Factors in selecting which test to use include the reason for testing, test availability, and cost. Generally, it is not recommended to test a person with both a TST and an IGRA.
If a person does not have TB disease, but has TB bacteria in the body, then latent TB infection is diagnosed. The decision about treatment for latent TB infection will be based on a person’s chances of developing TB disease.
People suspected of having TB disease should be referred for a medical evaluation, which will include