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U.S. Department of Health & Human Services

 

A person given the TB skin test must have a trained health care worker examine their forearm within 48 to 72 hours. If the person does not return within 72 hours, the test results are not valid and the person will need another skin test.

To determine whether a TB skin test reaction should be considered positive, a health care worker needs to interpret the reaction based on:

1. Size of induration (measured in millimeters [mm])
2. Person’s risk of TB infection or the risk of progression to TB disease if infected

Redness around the injection site is not measured. This is because the presence of redness alone does not indicate that a person has TB infection.

Measuring induration. In this photograph, the induration is 11 mm.