T-SPOT.TB: an in vitro diagnostic assay measuring T-cell reaction to Mycobacterium tuberculosis-specific antigens

The overall sensitivity of the RD1 ELISPOT assay (T-SPO.TB), which counts T cells sensitized to specific Mycobacterium tuberculosis peptide sequences, was 81-97% in populations with confirmed tuberculosis, in whom 39-78% had immunosuppressive conditions. In patients with confirmed tuberculosis, the respective RD1 ELISPOT versus tuberculin skin test (TST) sensitivities were 100% versus 89% in adults and 77% versus 35% in children (of whom 39% were HIV-positive). In contrast to that of the TST, the sensitivity of the RD1 ELISPOT assay was not significantly affected by age <36 months, or the immunological or nutritional status of the subjects. Specificity was 100% in two UK-based studies. Isolated false positives have been recorded in patients infected with non-tuberculous M. kansasii. A study investigating latent tuberculosis infection found no significant difference in results between HIV-positive and -negative participants for the RD1 ELISPOT assay, while the TST varied significantly with HIV status. Contact-tracing studies have demonstrated concordance between the RD1 ELISPOT assay and the TST of 65-89%. There is a significant correlation between a positive result and the degree of exposure to the index case for the RD1 ELISPOT assay, but not for the TST, in contact-tracing studies. Unlike the TST, the RD1 ELISPOT assay is not confounded by bacille Calmette-Guérin vaccination.