The name originally given by Koch in 1890 to a preparation derived from the tubercle bacillus Mycobacterium tuberculosis and intended for the diagnosis or treatment of TUBERCULOSIS. The basis of the tuberculin reaction is that any person who has been infected with the Mycobacterium tuberculosis, however long ago, has a visible skin reaction when a small amount of tuberculin is injected into the skin. A negative reaction means either that the individual has never been infected with the tubercle bacillus, or that the infection has been too recent for sensitivity to have developed. There are various methods of carrying out the test: for example, the Mantoux test is performed by injecting tuberculin into the skin on the forearm. The size of any raised, red area that results is a guide to the extent of possible infection.