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.
The two-step skin testing is not used in contact investigations or in other circumstances in which ongoing transmission of M. tuberculosis is suspected.