If an initial skin test is classified as negative, a second skin test should be administered 1-3 weeks after the first result was read.

If the second test is positive, it probably represents a boosted reaction, from a past infection. Response to tuberculin decreases over time. The initial TST stimulates the immune system, so that there is an immune response to a subsequent TST.

If the second test is negative, the person is classified as not infected.

Two step testing eliminates the false negative test results due to a weakened immune system.