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, an d 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 c linical, 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 furth er evaluation, 79. Among these patients, TST of 5 mm or larger was signific antly more likely to indicate active or inactive TB (p < 0.001). However, a mong patients with TST of 5 mm or greater, the size and frequency distribut ion of tuberculin reactions were not different between subjects with differ ent diagnoses, nor between subjects with different types or extent of radio graphic findings. As well, TST reactions were no different in 121 subjects with or 176 subjects without a history of BCG vaccination. In close contact s or patients suspected of active TB, reactions less than 5 mm indicated lo wer likelihood of active or inactive disease, but above that threshold, siz e of tuberculin reaction did not matter.