Evaluation of an interferon-gamma release assay, T-SPOT.TB, in a population with a low prevalence of tuberculosis.

Setting: Great Lakes, Illinois, USA.

Objective: To compare the performance of an interferon-gamma release assay (T-SPOT.TB) and tuberculin skin test (TST) in a population with a low prevalence of tuberculosis (TB) that was predominantly US-born and not bacille Calmette-Guérin-vaccinated.

Design: A total of 414 subjects with absence of a previous positive TST were enrolled, of whom 326 yielded analyzable results for both TST and T-SPOT.TB.

Results: Overall agreement between T-SPOT.TB and TST was 98.2% (95%CI 96.0-99.3). The specificity of T-SPOT.TB in individuals judged to be at low risk for TB infection was 98.9% (95%CI 96.9-99.8). Of 326 subjects, 8 (2.5%) had a positive T-SPOT.TB result, six of which occurred in the absence of a positive TST. Of these, at least three appeared to have risk factors, suggesting the possibility of a false-negative TST result.

Conclusion: Because of the excellent agreement between the TSPOT.TB and the TST, either test can serve as an effective diagnostic tool in populations at low risk for TB. As the tests have specific advantages and disadvantages, health care providers have leeway in choosing the most appropriate test for the population they are treating.