A negative reaction (no induration) or a level of induration below the cutoff for each risk group may indicate lack of infection with the bacteria which causes TB. There are different cutoffs for children, for people with HIV, and other risks groups.

Unfortunately, this is not a perfect test, and up to 20% of people infected with tuberculosis may not have a reaction on the PPD skin test. In addition, individuals with certain conditions which affect the immune system (cancer, recent chemotherapy, late-stage AIDS) may also have a false negative test result.

Significance depends on the size of the reaction on the skin and the individual. The reaction is measured in millimeters of induration (hard swelling) at the injection site.

A small reaction (5 mm) is considered to be positive in individuals with HIV, in individuals on steroid therapy, or in individuals in close contact with a person with active tuberculosis.

Larger reactions (greater than or equal to 10 mm) are considered positive in individuals with diabetes, renal failure and health care workers, among others. In individuals with no known risks for tuberculosis, a positive reaction requires a 15 mm or greater induration.