Objective: To analyse the performance of a new M tuberculosis-specific interferon γ (IFNγ) assay in patients with chronic inflammatory diseases who receive immunosuppressive drugs, including tumour necrosis factor α (TNFα) inhibitors.
Methods: Cellular immune responses to the M tuberculosis-specific antigens ESAT-6, CFP-10, TB7.7 were prospectively studied in 142 consecutive patients treated for inflammatory rheumatic conditions. Results were compared with tuberculin skin tests (TSTs). Association of both tests with risk factors for latent M tuberculosis infection (LTBI) and BCG vaccination were determined and the influence of TNFα inhibitors, corticosteroids, and disease modifying antirheumatic drugs (DMARDs) on antigen-specific and mitogen-induced IFNγ secretion was analysed.
Results: 126/142 (89%) patients received immunosuppressive therapy. The IFNγ assay was more closely associated with the presence of risk factors (odds ratio (OR) = 23.8 (95% CI 5.14 to 110) vs OR = 2.77 (1.22 to 6.27), respectively; p = 0.009), but less associated with BCG vaccination than the TST (OR = 0.47 (95% CI 0.15 to 1.47) vs OR = 2.44 (0.74 to (8.01), respectively; p = 0.025). Agreement between the IFNγ assay and TST results was low (κ = 0.17; 95% CI 0.02 to 0.32). The odds for a positive IFNγ assay strongly increased with increasing prognostic relevance of LTBI risk factors. Neither corticosteroids nor conventional DMARDs significantly affected IFNγ responses, but the odds for a positive IFNγ assay were decreased in patients treated with TNFα inhibitors (OR = 0.21 (95% CI 0.07 to 0.63), respectively; p = 0.006).
Conclusions: These results demonstrate that the performance of the M tuberculosis antigen-specific IFNγ ELISA is better than the classic TST for detection of LTBI in patients receiving immunosuppressive therapy for treatment of systemic autoimmune disorders.
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