Objective: To analyse the performance of a new M. tuberculosis (Mtb) specific interferon gamma (IFN-γ) assay in patients with chronic inflammatory diseases, who receive immunosuppressive drugs, including TNF-α inhibitors.
Methods: Cellular immune responses to the Mtb specific antigens ESAT-6, CFP-10, TB7.7 were prospectively studied in 142 consecutive patients treated for inflammatory rheumatic conditions. Results were compared to tuberculin skin tests (TST). Association of both tests with risk factors for LTBI and BCG vaccination were determined and the influence of TNF-α inhibitors, corticosteroids, and DMARDs on antigen-specific and mitogen-induced IFN-γ secretion was analysed.
Results: 126/142 (89%) patients received immunosuppressive therapy. The IFN-γ assay was more closely associated with the presence of risk factors (p=0.009), but less associated with BCG vaccination than the TST (p=0.025). Agreement between the IFN-γ assay and TST results was low (kappa 0.17; 95%-CI 0.02 to 0.32). The odds for a positive IFN-γ assay strongly increased with increasing prognostic relevance of LTBI risk factors. Neither corticosteroids nor conventional DMARDs significantly affected IFN-γ responses, but the odds for a positive IFN-γ assay were decreased in individuals treated with TNF-α inhibitors (p=0.006).
Conclusions: These results demonstrate a superior performance of the M.tuberculosis antigen specific IFN-γ ELISA compared to the classic TST for detection of LTBI in patients receiving immunosuppressive therapy for treatment of systemic autoimmune disorders.