Comparison testing of current (PPD-SI) and proposed (PPD-S2) reference tuberculin standards

Since 1951, the tuberculin PPD-S1 has been used to standardize commercial P PD reagents and perform special tuberculin surveys. PPD-S1 is now in short supply and a new standard (PPD-S2) has been manufactured. To determine if P PD-S2 is equivalent and can replace PPD-S1, we conducted a double-blind cli nical trial. between May 14 and October 28, 1997, 69 subjects with a histor y of culture-proven tuberculosis (TB patients) and 1,189 subjects with a ve ry low risk for TB infection were enrolled, received four skin tests (with PPD-S1, PPD-S2, and one each of the commercially available PPDs), and had r eactions measured by two trained observers. Among the TB patients, we found statistically indistinguishable immunogenicity (mean reaction size +/- sta ndard deviation): 15.6 +/- 6.6 mm for PPD-S1 and 14.8 +/- 5.6 mm for PPD-S2 . Among low-risk subjects, the tests had equally high specificities (PPD-S1 , 98.7% and PPD-S2 98.5%), using a 10-mm cutoff. The number of discordant ( negative versus positive) interpretations for PPD-S2, assuming that low-ris k subjects who had a greater than or equal to 10 mm reaction to PPD-S1 were truly infected, was low (0.5%) and indistinguishable from the rate of disc ordant interpretations of the same test when read by two different observer s (0.8%). The study results indicate that PPD-S2 is qualified to be used as the new U.S. reference standard for PPD tuberculin.