Problems connected with estimating the incidence of tuberculosis infection.

Many problems have to be faced in the estimation of an apparently simple but valuable index-namely, the incidence of tuberculosis infection. Very little attention seems to have been paid to these problems so far.Records from 50 villages in a district of South India, whose populations were tested with 1 TU of PPD RT 23 in Tween 80 diluent and retested after 18 months, have been examined for a reappraisal of existing methods. As a result, it has been found that some of these methods are subject to gross errors and that available figures are unreliable.For estimating the newly infected, a new approach based on the drawing of a curve for the distribution of differences in reaction size from one round of tuberculin testing to another is presented. Further, it is shown that the newly infected probably constitute a homogeneous group with an increase in mean reaction size of about 24 mm and standard deviation of 4 mm. Accordingly, 98% of the newly infected show an increase in reaction size of 16 mm or more. There are others who show similarly large increases in allergy on a retest, even in the absence of infection. The number of persons in the latter category rises with age and is likely to be greater in areas with a high prevalence of non-specific allergy.