Definition of PPD skin test:

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

How the test is performed:

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 at that time to have the area checked. This check 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 of infection.

How to prepare for the test:

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.

How the test will feel:

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

Why the test is performed:

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