Tuberculin Skin Test The tuberculin skin test is used to determine if someone has been exposed to tuberculosis. The purified protein derivative (PPD) is an antigen used to aid in the diagnosis of tuberculosis infection. An infection with the bacteria that causes tuberculosis frequently leads to a sensitivity to these antigens. The test site (usually the forearm) is cleansed. The PPD extract is then injected into the most superficial layer under the skin resulting in a blister on the skin. Because the reaction will take 48-72 hours to develop, your child must return to the doctor within that time for a proper evaluation of the test site. This will determine whether your child has had a significant reaction to the PPD test. A reaction is measured in millimeters of induration (hard swelling) at the site.