PPD skin test

Definition

The 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.

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 blister on the skin.

Because the reaction will take 48 - 72 hours to develop, you must return to your health care provider within that time for a proper evaluation of the test site. This will 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. Notify your health care provider if you are taking certain drugs such as steroids or have a medical condition that can affect your immune system such as cancer. Your PPD may be falsely negative or the standards for interpreting a reaction may be different.

How the test will feel

There will be a brief sting as a needle is inserted just below the skin surface.

Why the test is performed

The test is performed to evaluate infection with tuberculosis.

Normal Values

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 TB. 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.

What abnormal results mean

Significance depends on the size of the reaction on the skin and the individual. The reaction is measured in millimeters of induration (hard swelling) at the injection site.

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.

What the risks are

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.

Special considerations

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.

Illustrations

Tuberculosis in the kidney

Tuberculosis in the lung

Positive ppd skin test

PPD skin test

Page Content:

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