Test Frequency
Two-step testing has been recommended, involving a baseline and a retest. This is to rule out the "booster phenomenon," which may occur if many years have passed since an individual first became infected. (Immune reaction wanes over time, especially in older people.) The first TST result may be read as negative. In contrast, the second baseline (TST 1–3 weeks later) is positive, leading to a correct interpretation of TB infection but an incorrect interpretation as a recent conversion. More likely, the boosted reaction occurred because the first TST stimulated an old TB infection in the body.
Two-step testing should be used for the initial TST for individuals who will be retested periodically. In this scenario, a positive TST (after a negative baseline and retest) would indicate the individual likely has a TB infection. This result could be due to a conversion after exposure to TB.
The individual must be further evaluated (old infection or recent conversion).