Tuberculin

Tuberculin', currently named Purified Protein Derivative (PPD) is an antigen used to aid in the diagnosis of tuberculosis infection. An infection with the bacteria that causes tuberculosis frequently leads to a sensitivity to these antigens.

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 blister on the skin. Because the reaction will take 48-72 hours to develop, the patient must return to the health care provider within that time for a proper evaluation of the test site. This will determine whether he or she has had a significant reaction to the PPD test. A reaction is measured in millimeters of induration (hard swelling) at the site.

There is no special preparation for this test. The patient should inform the health care provider if he or she has ever had a positive PPD skin test. If so, the PPD test should not be repeated. The test should also be not performed if the pacient is taking some drugs, such as steroids or has a medical condition which can affect the immune system such as cancer. The PPD test may be falsely negative or the cutoffs when interpreting a reaction may be different.

A negative reaction (no induration) or a level of induration below the cutoff for each risk group may indicate lack of infection with the bacteria which causes tuberculosis. There are different cutoffs for children, for people with HIV, and other risks groups.

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

The test's significance depends on the size of the reaction on the skin and the individual. A small reaction (5 mm) is considered to be positive in individuals with HIV, in individuals on steroid therapy, or in individuals in close contact with a person with active tuberculosis. Larger reactions (greater than or equal to 10 mm) are considered positive in individuals with diabetes, renal failure and health care workers, among others. In individuals with no known risks for tuberculosis, a positive reaction requires a 15 mm or greater induration. This test is widely used. However, its limitations include the fact that there are false negatives and false positives. In addition, a positive skin test does not necessarily mean that an individual has active tuberculosis. Additional studies are needed to verify whether active disease is present.

There is a very small risk of severe redness and swelling of the arm in individuals who have had a previous positive PPD test and who undergo repeat testing. There have been a few cases of this reaction also occurring in individuals who have not been previously tested.

Tuberculin skin test. Medline Plus Medical Encyclopedia. Update Date: 8/11/2003. By: Kenneth Wener, M.D., Department of Infectious Diseases, Beth Israel Deaconess Medical Center, Boston, Maryland, USA. Review provided by VeriMed Healthcare Network. US Federal Government public domain.