Medical professionals read a PPD or TB skin test by viewing and measuring the test injection site; if a firm red bump develops at the site within two to three days of injection, it indicates previous exposure to tuberculosis, according to WebMD. Doctors use the test as a diagnostic tool for ruling out or confirming TB exposure.
The tuberculin skin test involves injecting purified protein derivative, or PPD, antigens just beneath the top layer of skin on the inner forearm, notes WebMD. Previous exposure to TB in the past causes a reaction, resulting in the development of a firm red bump.
If no bump forms or if a bump smaller than 0.2 inches forms, the test is negative, notes WebMD. A firm bump larger than 0.2 inches in patients at high risk for TB or 0.4 inches for people at moderate risk for TB is a positive result. In low-risk patients, a firm bump of 0.6 inches is positive for TB exposure.
Doctors typically order tuberculin skin testing due to the patient’s possible exposure to TB or if the patient exhibits symptoms of TB, such as a persistent cough, unexplained weight loss and night sweats. Doctors may also order the test if the patient has an abnormal chest X-ray indicating the potential presence of TB or for a patient who has an impaired immune system or a recent organ transplant.