There is relative dearth of data addressing this very important issue. A decreased IG response after mitogen stimulation during treatment with oral prednisolone has been reported (1).
In a study of patients with autoimmune disorders, 27% of patients on daily prednisolone dose of 10 mg or more had indeterminate QuantiFeron Gold In-Tube test compared to 1% of patients not taking prednisolone (1).
A meta-analysis of the performance of IGRAs (including T-SPOT.TB) concluded that these assays were negatively affected by immunosuppressive therapy (2). So, until more data becomes available, caution is advised in interpreting lack of positive test or indeterminate results of IGRAs in patients on corticosteroid therapy.
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1. Belard E, Semb S, Ruhwald M, et al. Prednisolone treatment affects the performance of the QuantiFERON Gold In-Tube test and the Tuberculin skin test in patients with autoimmune disorders screened for latent tuberculosis infection. Inflamm Bowel Dis 2011;17:2340-2349. https://academic.oup.com/ibdjournal/article/17/11/2340/4631016
2. Shahidi N, Fu Y-T, Qian H, et al. Performance of interferon-gamma release assays in patients with inflammatory bowel disease: a systematic review and meta-analysis. Inflamm Bowel Dis 2012;18:2034-2042. https://www.researchgate.net/publication/265093409_Performance_of_Interferon-gamma_Release_Assay_for_Tuberculosis_Screening_in_Inflammatory_Bowel_Disease_Patients