What indicates a positive reaction in a skin test for tuberculosis?

A positive reaction in a skin test for tuberculosis, specifically the Mantoux test, is indicated by redness and swelling at the injection site. This reaction is measured in millimeters and signifies that the immune system has been sensitized to the tuberculin antigen, indicating exposure to Mycobacterium tuberculosis.

When a person has been exposed to the bacterium, their immune system produces a response that results in a localized inflammatory reaction. The degree of induration (hardening) and accompanying redness can help healthcare professionals determine whether the test result is positive or negative.

In contrast, increased body temperature, itching at the injection site, and the formation of pustules do not serve as indicators of a positive tuberculin skin test. Increased body temperature can occur for various reasons, including other infections, and does not specifically correlate with tuberculosis exposure. Itching may arise from irritation or allergic reactions but is not an indicator of tuberculosis. The formation of pustules is not typical in a standard tuberculin skin test and may suggest another type of skin reaction or infection, rather than indicating a positive tuberculosis reaction.