 [ Symptoms ] what does a positive tb test look like [ Disease ] 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 do not count the cdc recommends different cutoffs for a positive tb skin test depending on a person is 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 do not 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