A purified protein derivative skin test is used to diagnose a tuberculosis infection, according to MedlinePlus. The test indicates whether or not a person has ever been in contact with bacteria that causes TB, a highly contagious lung disease.
The PPD test is performed over two visits to the doctor’s office, informs MedlinePlus. During the first visit, a small solution containing PPD is injected below the skin on the inside of the forearm. The injection forms a small bump that dissipates as the material is absorbed. The second visit occurs 24 to 72 hours later for a medical professional to determine if there was a strong reaction to the test.
PPD skin tests are interpreted based on the presence or absence of swelling at the injection site, explains MedlinePlus. A negative reaction to the PPD test shows little or no swelling at the injection site and means that an individual has never been infected with TB. A positive reaction to the PPD test shows swellings that are 5 millimeters to 10 millimeters in diameter.
Individuals who test positive with a PPD skin test do not necessarily have active TB, advises the Centers for Disease Control and Prevention. Additional tests are required to determine the active status of the disease. Individuals with active TB are treated to prevent later recurrence of the disease.