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The purified protein derivative is an antigen used to aid in the diagnosis of tuberculosis infection. This test is based on the observation that infection with the bacteria that causes tuberculosis frequently leads to a sensitivity to these antigens.

How the Test is Performed

The test site (usually the forearm) is cleansed. The PPD extract is then injected into the most superficial layer under the skin resulting in a transient bleb on the skin. As the reaction will take 48-72 hours to develop, you must return to your health care provider at 48-72 hours for a proper evaluation of the test site to determine whether you have had a significant reaction to the PPD test. A reaction is measured in millimeters of induration (hard swelling) at the site.

How to Prepare for the Test

There is no special preparation for this test. Inform your health care provider if you have ever had a positive PPD skin test -- if so, you should not have a repeat PPD test. Also, notify your health care provider if you are taking certain drugs such as steroids or have a medical condition which can affect your immune system such as cancer, as your PPD may be falsely negative or the cutoffs when interpreting a reaction may be different.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age: