What is two-step testing?
If it has been many years since a person was infected with TB, his or her initial skin test may be negative, because of waning immunity. Subsequent tests may be positive, however, because the initial tuberculin placement stimulates the immune response to the test. This phenomenon is referred to as the “booster effect.” The booster effect can be misinterpreted as a new skin test conversion (i.e. a recent TB infection). To avoid this problem, two-step testing should be used as the initial test in people where repeat testing is anticipated (such as hospital workers or nursing home residents). To perform two-step testing, a second skin test is placed 1-3 weeks after the initial skin test, if the first test is negative. If the second test remains negative, the person is presumed to be negative for latent tuberculosis, and any subsequent positive tests are the result of new infection. If the second test turns positive, this is a boosted response from prior TB infection. The person should be
