What Does Positive Vs. Negative Look Like on a TB Test?

The standard test for TB is a skin test in which a small amount of PPD, or purified protein derivative, is injected just below the skin, usually on the forearm. A raised, hardened, swollen or palpable area at the injection site is positive, says the Centers for Disease Control and Prevention.

Test results are read within 48 to 72 hours. Also known as a Mantoux test, the PPD test determines if a person has tuberculosis or has come into contact with the bacteria that causes TB. Tuberculosis is easily spread and can remain dormant for many years. Most people in the United States who are infected with the bacteria do not have signs or symptoms of TB, says MedlinePlus.

A positive test does not necessarily mean active TB. More tests, such as a chest X-ray or sputum test, must be done to determine whether there is active disease, MedlinePlus advises. People most likely to need a TB test are those who have been around someone with TB, health care workers, nursing home residents and people with a weakened immune system. Persons who have had a positive PPD test in the past should inform their health care provider. People from countries where TB is common may have received the BCG vaccine against TB and may have a positive reaction to a TB skin test.