most people being screened for tuberculosis get the tuberculin skin test also called the purified protein derivative (ppd). a small amount of harmless disintegrated tuberculosis protein is injected under the skin on the forearm. within a few days most people who have been previously infected with tuberculosis will develop a raised firm (indurated) area on the arm. redness and bruising don’t count. the cdc recommends different cutoffs for a positive tb skin test depending on a person’s risk for tuberculosis. for people with no risk factors for tuberculosis an indurated area 15 millimeters across is considered positive. as little as 5 millimeters of induration is a positive test for someone with an impaired immune system or a known tb exposure. a trained reader needs to interpret the tb test. most people with a positive tb skin test don’t recall symptoms of tuberculosis. they may have a normal chest x-ray as well. nevertheless they still likely have a small number of tuberculosis bacteria hiding somewhere in the lungs. they have about a 10% lifetime risk of developing full-blown tb called reactivation tuberculosis. to prevent this antibiotic treatment may be recommended after a positive tb test depending on age and other medical conditions.
what does a positive tb test look like?