PPD skin test Health Article

Definition

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

Alternative Names

Purified protein derivative standard; TB skin test; Tuberculin skin test

How the test is performed

The test site (usually the forearm) is cleansed. The PPD extract is then injected under the top layer of skin, causing a blister to form on the skin.

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 will determine whether you have had a significant reaction to the PPD test. A reaction is measured in millimeters of hard swelling (induration) at the site.

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.

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 test is done to find out if you have been infected with tuberculosis.

Normal Values

A negative reaction (no induration) or a level of hard swelling that falls below the cutoff for each risk group may mean that you have not been infected with the bacteria that cause TB. There are different cutoffs for children, people with HIV, and other risk groups.

Unfortunately, this is not a perfect test. Up to 20% of people infected with tuberculosis may not have a reaction on the PPD skin test. In addition, certain conditions that affect the immune system (cancer, recent chemotherapy, late-stage AIDS) may cause a false-negative test result.