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Open Access Highly Accessed Research article

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

Dag Gundersen Storla14*, Ingun Kristiansen2, Fredrik Oftung3, Gro Ellen Korsvold3, Monica Gaupset2, Gerd Gran5, Anne Kristin Øverby6, Anne Margarita Dyrhol-Riise5 and Gunnar Aksel Bjune1

Author Affiliations

1 Department of International Health, Institute of General Practice and Community Medicine, University of Oslo, PO Box 1130 Blindern, N-0318 Oslo, Norway

2 Center for Health Promotion, Akershus University Hospital, Sykehusveien 27, N-1478 Lørenskog, Norway

3 Division of Infectious Disease Control, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, N-0403 Oslo, Norway

4 Competence Centre for Imported and Tropical Diseases, Ullevål University Hospital, Kirkeveien 166, N-0450 Oslo, Norway

5 Department of Medicine, Haukeland University Hospital, Jonas Lies vei 65, N-5053 Bergen, Norway

6 Tuberculosis Control Unit, Ullevål University Hospital, Kirkeveien 166, N-0450 Oslo, Norway

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BMC Infectious Diseases 2009, 9:60  doi:10.1186/1471-2334-9-60

Published: 11 May 2009

Abstract

Background

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.

Methods

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-γ release assays based on the M. tuberculosis-specific ESAT-6 and CFP10 antigens, to investigate if this test might improve infection control measures.

Results

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.

Conclusion

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-γ release assays.