QuantiFERON-TB (QIFN) (CSL Limited) is a whole-blood assay for the recognition of infection with Mycobacterium tuberculosis. QIFN measures gamma interferon (IFN-gamma) production when purified protein derivatives (PPDs) of mycobacteria are incubated with venous blood samples. The specificity of QIFN in medical students before and after BCG immunization was assessed, and sensitivity in patients with tuberculosis was assessed. Antigens were PPD derived from M. tuberculosis and two M. tuberculosis-specific proteins, ESAT-6 and MPT-64. Of 60 medical students, all of whom had 0-mm tuberculin skin tests (TSTs) at study entry, 58 (97%) were initially classified as negative for M. tuberculosis infection by PPD QIFN. Five months after BCG immunization, 7 of 54 students (13%) had a TST result of >/=10 mm and 11 of 54 students (20%) tested positive by PPD QIFN. ESAT-6- and MPT-64-stimulated IFN-gamma responses in the medical students were negative prior to and after BCG immunization. For patients with active tuberculosis, 12 of 19 (63%) were positive by PPD QIFN, 11 of 19 (58%) were positive by ESAT-6 QIFN, and 0 of 12 were positive by MPT-64 QIFN. In conclusion, PPD QIFN was negative in 97% of a low-risk population who had not received BCG and who had negative TSTs. The specificities of both the TST and PPD QIFN were reduced following BCG immunization. PPD QIFN and ESAT-6 QIFN were of similar and moderate sensitivity in patients with active tuberculosis, but ESAT-6 QIFN is likely to be more specific because it is not influenced by past BCG exposure.