Tuberculin Skin Test

The PPD skin test is a method used to help with the diagnosis of a disease called tuberculosis (TB).

HOW THE TEST IS DONE

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.

PREPARATION FOR TEST

There is no special preparation for this test. People with a skin rash or other skin irritations on their arms may need to have the test performed at a different spot on the body.

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.

NORMAL FINDINGS

A negative reaction (no induration) or a level of hard swelling that falls below a certain cutoff may mean that a person has 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, and 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 reaction will take 48 - 72 hours to develop. You must return to your health care provider within that time to have the area checked. Follow your caregiver's instructions as to where and when to report for this to be done.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.

WHAT ABNORMAL RESULTS MEAN

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, or 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, and in health care workers, among others. In people with no known risks for tuberculosis, a positive reaction requires 15 mm or more of hard swelling at the site.

RISKS AND COMPLICATIONS

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 rare cases of this reaction in people who have not been tested before.

CONSIDERATIONS

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

MEANING OF TEST

Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.

OBTAINING THE TEST RESULTS

It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.