procedure for the diagnosis of tuberculosis infection by the introduction into the skin (skin test), usually by injection on the front surface of the forearm, of a minute amount of purified protein derivative (PPD) tuberculin. Tuberculin is a protein substance from the tuberculosis-causing bacillus, Mycobacterium tuberculosis, first discovered and extracted by Robert Koch in 1890. When the test is positive, a region of swelling 10 mm (0.4 inch) or greater in diameter, usually accompanied by redness, occurs within 48 hours at the site of injection. A positive reaction indicates that the individual was previously exposed to the tubercle bacillus, but it does not necessarily indicate that active clinical tuberculosis is present, or ever existed. The test is a help to the physician in determining the source and time of an infection and in distinguishing tuberculosis from other pulmonary conditions.