The PPD skin test is a method used to diagnose tuberculosis (TB). PPD stands for purified protein derivative.

The PPD test is done to find out if you have been infected with Mycobacterium tuberculosis, the bacteria that causes TB.

The test site (usually the forearm) is cleansed with rubbing alcohol. The PPD extract is then injected under the top layer of skin, causing a welt to form on the skin. This welt usually goes away in a few hours.

The reaction will take 48 - 72 hours to develop. You must return to your health care provider within that time to have the area checked. This test will determine whether you have had a significant reaction to the PPD test. The reaction is measured in millimeters of firm swelling (induration), not redness, at the site.

There is no special preparation for this test.

Tell your health care provider if you have ever had a positive PPD skin test. If so, you should not have a repeat PPD test, except under unusual circumstances.

Tell your doctor if you have a medical condition or if you take certain drugs, such as steroids, that can affect your immune system. These situations may lead to inaccurate test results.

You will feel a brief sting as a needle is inserted just below the skin surface.