Interferon-Gamma Assay T-SPOT.TB in the Diagnostics of Latent Tuberculosis Infection

Abstract

The diagnosis of latent tuberculosis infection (LTBI) is currently based on the century-old tuberculin skin test (TST). However a positive reaction can result from infection by Mycobacterium tuberculosis, BCG vaccination or cross-reaction with nontuberculous mycobacteria. T-SPOT.TB assay is a new test to diagnose tuberculosis infection by measuring in vitro T-cell interferon gamma release in response to two Mycobacterium tuberculosis-specific antigens: ESAT-6 and CFP 10. T-SPOT.TB assay has been performed on whole blood samples (n = 137) from March to September 2010. A tuberculin skin test result was available for 96 of participants. A positive TST result was considered if the induration was 10 mm or more. Of the 137 patients tested, T-SPOT.TB assay results were positive in 37 (27%), negative in 98 (71.5%) and indeterminate in only 2 (1.5%) persons. We analyzed T-SPOT.TB and TST results in the 96 patients for whom both test were available. Concordance between T-SPOT.TB and TST results (10 mm skin reaction interpreted as positive) was 79%. Fifteen (15.6%) patients had a positive TST result and a negative T-SPOT.TB and 5 (5.2%) patients had a negative TST result and a positive T-SPOT.TB. We observed good correlation between positive T-SPOT.TB results and the size of induration ≥ 15 mm in TST results. T-SPOT.TB offers a more accurate approach than TST for identification tuberculosis infection. The study shows that the test T-SPOT.TB is a good diagnostic tool in identifying persons with tuberculosis infection. For full confirmation of this assessment, it is necessary to examine more cases.