PPD skin test
The PPD skin test is a method used to diagnose tuberculosis (TB). PPD stands for purified protein derivative.
Purified protein derivative standard; TB skin test; Tuberculin skin test; Mantoux test
How the test is performed
The test site (usually the forearm) is cleansed with rubbing alcohol. The PPD extract is then injected under the top layer of skin, causing a welt to form on the skin. This welt usually goes away in a few hours.
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 test will determine whether you have had a significant reaction to the PPD test. The reaction is measured in millimeters of firm swelling (induration), not redness, 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, except under unusual circumstances.
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 PPD test is done to find out if you have been infected with Mycobacterium tuberculosis, the bacteria that causes TB.
A negative reaction (no induration) or a size of hard swelling that falls below the cutoff for each risk group may mean that you have 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. Up to 20% of people infected with the bacteria that cause TB 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.
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 firm swelling at the site) is considered to be positive in people:
- Who have HIV
- Who are taking steroid therapy
- Who have been in close contact with a person who has active TB
Larger reactions (greater than or equal to 10 mm) are considered positive in:
- People with diabetes or kidney failure
- Health care workers
- Injection drug users
In people with no known risks for TB, a positive reaction means there is 15 mm or more of firm swelling at the site.
What the risks are
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 cases of this reaction in people who have not been tested before.
A positive skin test does not necessarily mean that a person has active tuberculosis. More tests must be done to check whether there is active disease.
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. However, many experts believe that a past BCG vaccination should not change the interpretation of the PPD result when the test is done 15 years or more after the BCG vaccination.
Iseman MD. Tuberculosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 345.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Jatin M. Vyas, PhD, MD, Instructor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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