Definition of PPD skin test:

The PPD skin test is a method used to diagnose tuberculosis (TB). PPD stands for purified protein derivative.

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 at that time to have the area checked. This check 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 of infection.

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 cause TB.

Normal Values:

A negative reaction (no induration) or a size of hard swelling that falls below the cutoff for each risk group usually means 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.

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 impair the immune system (cancer, recent chemotherapy, late-stage AIDS) may cause a false-negative test result.

What abnormal results mean:

The test results depend on the size of the skin reaction and 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 have received an organ transplant
• Who have a suppressed immune system or are taking steroid therapy (about 15 mg of prednisone per day for 1 month)
• Who have been in close contact with a person who has active TB
• Who have changes on a chest x-ray that look like past TB

Larger reactions (greater than or equal to 10 mm) are considered positive in:

• People with a known negative test in the past 2 years
• People with diabetes, kidney failure, or other conditions that increase their chance of getting active TB
• Health care workers
• Injection drug users
• Immigrants who have moved from a country with a high TB rate in the past 5 years
• Children under age 4
• Infants, children, or adolescents who are exposed to high-risk adults
• Students and employees of certain group living settings, such as prisons, nursing homes, and homeless shelters

In people with no known risks for TB, 15 mm or more of firm swelling at the site indicates a positive reaction.

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.

Special considerations:

A positive skin test does not necessarily mean that a person has active TB. 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, most experts say that a past BCG vaccination should not change the PPD result when the test is done in people with an increased risk of TB infection or disease.