Evaluation of an In Vitro Assay for Gamma Interferon Production in Response to Mycobacterium tuberculosis Infections

The tuberculin skin test (TST) is the “gold standard” for detecting infection with Mycobacterium tuberculosis. We compared the TST using purified protein derivative to the QuantiFERON-TB test (QFT). Two groups were examined. Group 1 individuals (n = 66) (low risk) were at low risk for exposure to M. tuberculosis and were not Mycobacterium bovis BCG vaccinated. Group 2 (n = 29) include individuals who were likely to have been exposed to a high prevalence of M. tuberculosis infections and were BCG vaccinated. Group 1 individuals were given a TST. Group 2 individuals were not given a TST because of possible adverse reactions. A 10- to 15-mm indurated area 48 h after TST was considered positive. A positive QFT result was defined as a significant gamma interferon response to M. tuberculosis antigen, Mycobacterium avium antigen, and a nonspecific mitogen stimulus and no response in the negative control. In group 1, 60 of 66 individuals (90.9%) were negative by both methods, and 1 person was positive by both methods. There was one QFT-negative, TST-positive case, one QFT-positive, TST-negative case, and three conditional QFT-positive, TST-negative cases. In group 2, 12 of 29 (41.4%) were positive by QFT and considered likely to be TST positive because of prior BCG vaccination. QFT testing in our low-risk group resulted in an agreement of 96.8%, a sensitivity of 50%, and a specificity of 98.4% compared with TST results. QFT testing with TST in low-risk groups can aid in the detection of latent M. tuberculosis infections.