What does a positive skin test for tuberculosis indicate?

A positive skin test for tuberculosis, also known as the Mantoux test, primarily indicates previous exposure to the bacteria that cause tuberculosis (TB). When a person has been exposed to TB, their immune system develops a response that can be detected by the skin test. The test involves injecting a small amount of purified protein derivative (PPD) from the TB bacteria into the skin. If the individual has been exposed, their immune response will cause a localized swelling at the injection site after 48 to 72 hours.

It's important to note that a positive reaction does not necessarily confirm that the person has an active TB infection at that moment. It simply indicates that the immune system has recognized the TB proteins due to past exposure. Further evaluation, such as a chest X-ray or sputum test, would be necessary to diagnose an active infection.

In addition, a positive skin test does not indicate a full recovery from TB, nor does it reflect an allergic reaction to TB proteins; rather, it signifies that the immune system has encountered the bacterium in the past. Thus, recognizing the distinction between exposure and active disease is critical in interpreting the results of a TB skin test.