The PPD skin test is a method used to diagnose tuberculosis. PPD stands for purified protein derivative.
Purified protein derivative standard; TB skin test; Tuberculin skin test (also known as the Mantoux screening test, Tuberculin Sensitivity Test, Pirquet test, or PPD test for Purified Protein Derivative).
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 licensed nurse will evaluate your test site within 48 to 72 hours to determine if you have been exposed to tuberculosis. The reaction is read by measuring the diameter of induration (palpable raised hardened area) across the forearm (perpendicular to the long axis) in millimeters. If there is no induration, the result should be recorded as "0 mm". Erythema (redness) should not be measured.
False positive result. Due to the test's low specificity, most positive reactions in low-risk individuals are false-positives.A false positive result may be caused by nontuberculous mycobacteria or previous administration of BCG vaccine. Prior vaccination with BCG may result in a false-positive result for many years afterwards.
False negative result. Those that are immunologically compromised, especially those with HIV and low CD4 T cell counts, frequently show negative results from the PPD test.This is because the immune system needs to be functional to mount a response to the protein derivative injected under the skin..