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Q.

What does a positive TB test look like?

 

Answers From Experts & Organizations (1)

Internal Medicine
Emory University
131 Answers
50,621 Helpful Votes
46 Followers
A.

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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