Final answer:
For a TB skin test, also known as PPD test, 0.1 mL of tuberculin is injected into the skin, which is checked for a reaction after 48-72 hours. A raised, red induration at the injection site indicates a positive result, implying a previous exposure to the TB bacteria.
Step-by-step explanation:
In the Mantoux test, which is a standard method used in the United States, the injection is made on the inner surface of the forearm. After the administration, the test is read in 48-72 hours, with a positive reaction identified by redness, swelling, or hardness, also known as induration, at the site of injection. The size of the induration is then measured to determine the final result. A positive test indicates that the individual has been exposed to Mycobacterium tuberculosis and that their immune system has a cellular response to the bacterium.
It's worth noting that individuals vaccinated with the BCG vaccine may show a false-positive result because the vaccine introduces an immune response to similar antigens as those in the tuberculin skin test. In such cases, chest radiographs can be used to further assess and confirm exposure to TB, particularly if a person has a positive PPD test but does not show symptoms of active TB.