The significance of the tuberculin test using bioequivalent PPD

The biological reactivity of two commercially available tuberculin preparations was measured in 168 subjects (9 with active tuberculosis; 98 with inactive tuberculosis; and 61 general population). One tuberculin had 5 tuberculin units (TU) per 0-1 ml dose relative to a stabilized solution of PPD-S, whereas the other preparation was bioequivalent to 5 TU per 0-1 ml dose of unstabilized PPD-S, and was therefore of lower potency. Differences between the tuberculins were more evident when an induration of larger than or equal to 10 mm was considered as positive. Using a criterion of larger than or equal to 5 mm the differences were smaller, but the frequency of non-tuberculous responders was increased using this criterion. The average difference between the tuberculins was of the order of 5 mm regardless of the criterion of postivity. The use of chemoprophylaxis based on reaction size necessitates that the physician should know which of these two commercially available tuberculins is being used.