High reproducibility of the interferon-gamma release assay T-SPOT.TB in serial testing

Longitudinal studies regarding the reproducibility of Interferon-gamma release assay (IGRA) T-SPOT.TB for the diagnosis of Mycobacterium tuberculosis (M. tb) infection in serial testing are limited. We retrospectively analysed results of serially tested subjects in a medical laboratory in Germany over a time period of 14 years. From October 2004 to December 2018, a total of 5440 subjects were identified with a second T-SPOT.TB test after a median time interval of 258 days (interquartile range [IQR] 62-665). Consistently negative (n = 4520) or positive results (n = 682) were observed in 5202 (95.6%) subjects, indicating a high degree of concordance in serial testing (κ = 0.83). Test conversions occurred in 101 of 4621 (2.2%) subjects with initially negative tests. Of 819 subjects with initially positive test results, 137 (16.7%) had a test reversion which was associated with low spot numbers of the first test. Of 529 subjects retested within 1 year, only 60 (11.3%) displayed a test reversion. In subjects retested after more than 1 year, 77 of 290 (26.6%) tests reverted. This significantly higher rate of test reversions after more than 1 year was age-dependent and only observed in subjects above the age of 40 years. In the medical laboratory, the T-SPOT.TB test demonstrates a high reproducibility in serial testing.