Choosing an appropriate cutting point for conversion in annual tuberculin skin testing

A previous report from our institution showed that the use of a two-step tuberculin testing program in new employees reduced the apparent rate of conversion in annual skin testing by eliminating the effect of the booster phenomenon. However, the converter rate subsequently remained too high. We propose that the use of 10 mm of induration as the cutting point gave a high rate of false conversion in our area of high prevalence of sensitivity to nontuberculous mycobacteria. A change to 15 mm of duration as the cutting point reduced our conversion rate from 2.6 to 1.2%, identified a group who demonstrated continued large tuberculin reactions at repeat testing, and eliminated age dependency from our converter population. We propose the choice of 15 mm of induration as the appropriate cutting point in annual employee tuberculin skin testing programs in areas in which sensitivity to nontuberculous mycobacteria is common.