In the epidemiological surveys carried out by the National Tuberculosis Institute (NTI), Bangalore, instead of reading the tuberculin reactions (indurations) by measuring their transverse diameters as is done conventionally, the longitudinal diameters were read. Later on, as the longitudinal diameters were observed to be larger than the transverse diameters, an investigation was carried out to study whether this difference would affect the estimation of infection rates. Out of 1,240 tuberculin tested persons, for 1,189 both transverse and longitudinal diameters were read by each of two readers, one accustomed to read the longitudinal diameter and the other, the transverse diameter. All care was taken to avoid bias on the part of the readers. All four reading were available for 1,075 persons (87%).

It was found that longitudinal diameters were larger than the transverse diameters for all ranges (of sizes) of reactions when either diameter for each reader was taken as standard. The prevalence of infection, considering 10mm+ reactions as the minimum level for those infected, were almost the same for both the diameters and for both the readers. Analysis according to age sex gave similar results. Variations between the readers are known to be of much higher magnitude than those observed between the diameters in this study. The levels of post-vaccination allergy calculated on the basis of longitudinal diameters, however, will be larger than those for transverse diameters. In view of the above results, the findings of the NTI epidemiological surveys wherein longitudinal diameters of tuberculin reaction are read, will not only be comparable with other studies conducted by NTI but also with studies done by other organizations-national or international.