What is the purpose for 2-Step tuberculosis skin testing?
The 2-Step process is useful in uncovering the “Booster Phenomenon. Some people exposed to Mycobacterium tuberculosis (TB) may have an immune system that has “forgotten about” the exposure, and have an initially negative skin test reaction to tuberculin if the test is given many years afterwards. This skin test, however, may “remind” the immune system about the exposure (or “boost” the immune system’s ability to react to the tuberculin.) This can cause a positive reaction when the person is next tested, which would be then interpreted as a conversion indicating a new exposure.
Using a 2-step process will identify a positive skin test as a result of a remote history of previous TB exposure at the time of baseline testing; thus avoiding later positive skin tests being misinterpreted as new exposure. For this reason, the 2-step process is particularly useful for baseline testing of individuals (like nursing students) who will be working in a healthcare environment.
How is “Two-Step” testing done?
The baseline tuberculin test is applied and read as usual. If this test is negative, the individual has a repeat skin test in 1 to 3 weeks. If this is also negative, then the individual is considered uninfected, and is tested as usual in subsequent years. However, if the second test is positive, the individual should be considered infected and treated accordingly, but this would not be considered a conversion.