Use of interferon gamma-based assay to diagnose tuberculosis infection in health care workers after short term exposure

We intended to assess the risk for health care workers (HCWs) of acquiring M. tuberculosis infection after exposure to patients with sputum-smear positive pulmonary tuberculosis at three University Hospitals (Ullevål, Akershus, and Haukeland) in Norway. We tested 155 exposed health care workers and 48 healthy controls both with a tuberculin skin test (Mantoux) and the T-SPOT.TB test, a recently developed interferon-gamma release assays based on the M. tuberculosis-specific ESAT-6 and CFP10 antigens, to investigate if this test might improve infection control measures. Among the 155 exposed HCWs tested in this study, 27 individuals were defined as newly infected cases by TST after recent exposure, while only 3 of these had a positive T-SPOT.TB test. The number of T-SPOT.TB positives represents 11% of the individuals defined as recently infected by TST after exposure (3/27) and 2% of the total number of exposed people tested (3/155). In addition, 15 individuals had been previously defined as infected by TST before exposure of whom 2 subjects were T-SPOT.TB positive. All individuals detected as T-SPOT.TB positive belonged to the TST positive group (> 15 mm), and the percentage concordance between T-SPOT.TB and TST, including both previously and newly infected subjects, was 12% (5/42). The 48 control participants used in the study were all T-SPOT.TB negative, but 3 of these subjects were TST positive. Our data indicate that the frequency of latent TB in the total cohort of HCWs is 3%, whereas the rate of transmission of TB to exposed individuals is approximately 2% and occurs through exposure periods of short duration. Thus, the risk of TB transmission to HCWs following TB exposure in a hospital setting in Norway is low, and improved screening approaches will benefit from the application of specific interferon-gamma release assays.