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.
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