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

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.

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.

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

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

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.

The results of the test depend on the size of the skin reaction and on the person being tested.

A small reaction (5 mm of hard swelling at the site) is considered to be positive in people:

• Who have HIV
• Who are taking steroid therapy
• Who have been in close contact with a person who has active tuberculosis

Larger reactions (greater than or equal to 10 mm) are considered positive in:

• People with diabetes or kidney failure
• Health care workers

In people with no known risks for tuberculosis, a positive reaction requires 15 mm or more of hard swelling at the site.

There is a very small risk of severe redness and swelling of the arm in people who have had a previous positive PPD test and who have the test again. There also have been a few cases of this reaction in people who have not been tested before.

A positive skin test does not necessarily mean that a person has active tuberculosis. More tests will be done to check whether there is active disease.

Many people who were born outside the United States may have had a vaccine called "BCG," which can lead to a false-positive test result. However, many experts believe that a past BCG vaccination should not change the PPD result when the test is done 15 years after the vaccination.

Centers for Disease Control and Prevention. Questions and answers about TB. 2007. Accessed September 28, 2008.