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Aim: to compare the efficacy of infliximab in inflammatory bowel disease (IBD) and ankylosing spondylitis (AS) and its relationship with drug concentration and the level of antibodies against this medication. Patients and Methods: this prospective observational study included 34 patients with AS, Crohn’s disease, and ulcerative colitis who received infliximab for 7.5 [5; 21.5] months. Treatment efficacy was assessed in all patients by a single measurement of trough concentration of infliximab and serum antibodies (Abs) against this medication. Results: infliximab was reported to be clinically effective in 63.6% of patients with IBD and 83.3% of patients with AS. Abs against infliximab were identified in 22.7% of patients with IBD and 25% of patients with AS. Low infliximab concentration was detected in 31% of patients Пациенwith IBD and 50% of patients with AS. Meanwhile, no significant differences between the groups were reported. Among patients with antiinfliximab Abs, a good treatment effect was observed in two patients (25%) who received therapy for > 16 months. In patients with AS, infliximab inefficacy and infusion reactions were accounted for by the generation of Abs (66.7%) or low infliximab concentration (100%). Among patients with IBD, Abs and low infliximab concentration were identified only in 25% patients in whom infliximab was ineffective. In contrast to AS, certain factors affecting drug efficacy (i.e., hormone dependence and body mass index), its concentration (i.e., extraintestinal manifestation), and Ab levels (i.e., disease duration and body mass index) were identified in IBD. Conclusions: features identified that represent various mechanisms of immune escape and immunogenicity in inflammatory immunemediated disorders should be considered when extrapolating the data between disorders and developing management strategy for inadequate treatment response to infliximab. KEYWORDS: infliximab, ankylosing spondylitis, inflammatory bowel disease, trough concentration, antibodies against infliximab. FOR CITATION: Nuriakhmetova T.Yu., Valeeva I.Kh., Shevnina Ya.O. et al. Specificity of treatment response to infliximab in patients with inflammatory bowel disease and ankylosing spondylitis. Russian Medical Inquiry. 2021;5(2):64–70. DOI: 10.32364/2587-6821- 2021-5-2-64-70.