What Is PPD Skin Testing?
PPD Skin testing, also known as the tuberculin skin test (TST), is a method used to determine if someone has been exposed to the bacteria that cause tuberculosis (TB). During the test, a small amount of purified protein derivative (PPD) is injected just under the skin, usually on the forearm. After 48 to 72 hours, the person returns to have the injection site checked for any reaction. A raised, firm area at the site, known as induration, is measured to determine the result. The size of the induration, along with the person's medical history and risk factors, helps assess whether the test is positive or negative.
A positive PPD Test indicates that the individual has been exposed to the TB bacteria at some point, but it does not confirm active TB infection. The test cannot differentiate between a past infection or active disease. If the test is positive, further evaluation may be needed to assess the risk of active TB. Although the PPD test is widely used, it can sometimes produce false positives, especially in people who have received the BCG vaccine or have been exposed to other non-TB mycobacteria.
