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.