The PPD test evaluates a skin reaction caused by tuberculin administered to the inner surface of the forearm. The observation of an induration (hardening) with a diameter of 5 mm or larger at the application site indicates a positive test result. A positive PPD test suggests that the person has been infected with tuberculosis bacteria. A negative test result, on the other hand, generally indicates the absence of tuberculosis infection. Following a positive test result, the doctor may request additional examinations such as a chest X-ray, computed tomography (CT) scan, and sputum test to confirm the diagnosis and rule out active disease.