Diagnosis of patients with minimal active tuberculosis (TB) is difficult, as there is no single test with high sensitivity and specificity. The yield and clinical utility of a combination of diagnostic tests were prospectively studied among 500 consecutive patients referred for sputum induction for diagnosis of possible active TB. Patients underwent sputum induction, chest X-ray, tuberculin testing, and had blood drawn for serologic testing (Detect-TB test; Biochem ImmunoSystems). Sputum was examined with fluorescent microscopy and PCR (Amplicor MTB-Roche) and cultured for mycobacteria using liquid (BACTEC) and solid media. For the diagnosis of the 60 cases of active TB, sensitivity and specificity, respectively, of the following diagnostic tests were mycobacterial culture, 73% and 100%; PCR, 42% and 100%; chest X-ray, 67-77% and 66-76%; tuberculin testing, 94% and 20%; and serology, 33% and 87%. After consideration of PCR and radiographic and clinical characteristics, a positive serologic test was independantly associated with diagnosis of active disease (adjusted odds of disease if positive, 2.6; 95% confidence limits, 1.1,6.1). No currently available test has sensitivity and specificity high enough for the accurate diagnosis of minimal pulmonary TB. Utilization of a combination of tests, together with consideration of key clinical characteristics, could improve diagnostic accuracy.

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.