Purified protein derivative standard; TB skin test; Tuberculin skin test
Normal Values
A negative reaction (no induration) or a level 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 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.
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
- 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
- Health care workers
In people with no known risks for tuberculosis, a positive reaction requires 15 mm or more of hard swelling at the site.
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