Does size matter? Utility of size of tuberculin reactions for the diagnosis of mycobacterial disease.

It is a common belief that larger tuberculin reactions are more serious, and more likely to indicate patients with active tuberculosis (TB) or at high risk of disease in the future. Among 182 close contacts, and 502 patients suspected of possible active TB, 529 underwent tuberculin skin testing (TST) and 605 had a chest radiograph. Final diagnoses, based on all available clinical, microbiological, histological, and radiographic information, were active TB, 68; inactive TB, 274; nontuberculous mycobacterial disease, 14; conditions associated with anergy, 36; no detectable abnormality (except a positive TST) or condition unrelated to TB, 213; and negative TST, no further evaluation, 79. Among these patients, TST of 5 mm or larger was significantly more likely to indicate active or inactive TB (p < 0.001). However, among patients with TST of 5 mm or greater, the size and frequency distribution of tuberculin reactions were not different between subjects with different diagnoses, nor between subjects with different types or extent of radiographic findings. As well, TST reactions were no different in 121 subjects with or 176 subjects without a history of BCG vaccination. In close contacts or patients suspected of active TB, reactions less than 5 mm indicated lower likelihood of active or inactive disease, but above that threshold, size of tuberculin reaction did not matter.