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To assess the clinical significance of antibodies to infliximab (ATI) formation in patients with inflammatory bowel disease (IBD).Infliximab therapy in patients with IBD is highly effective though up to 50% of patients lose response to infliximab. ATI formation occurs in 6% to 60% of patients on scheduled infliximab maintenance therapy.Infliximab trough levels and ATI at trough were prospectively determined in patients with IBD on maintenance infliximab therapy. Patients on infliximab maintenance therapy with at least 3 ATI measurements were included. Patients were divided into 2 groups: sustained negative (1 µg/mL) ATI levels, and fluctuating ATI levels (at least 1 sample with ATI1 and at least one1 µg/mL).Forty-eight patients with IBD with available clinical data and serum samples were included. 25 patients had sustained low ATI levels and 23 patients had fluctuating ATI levels. Both groups were similar in IBD subtype distribution (Crohn's disease in 73.9% and 60%), mean serum albumin levels (4.2 vs. 3.9 g/dL), and mean trough serum infliximab levels (3.3 vs. 4.6 µg/mL) in fluctuating ATI and low ATI groups, respectively. There were no significant differences in the rate of clinical response (64% vs. 76%) and mean serum C-reactive protein levels (9.4 vs. 8.5 mg/dL, ULN=5) in the fluctuating ATI and low ATI groups. Similar rates of clinical response in the fluctuating ATI and low ATI groups were observed in ulcerative colitis (33% vs. 40%) and Crohn's disease (76% vs. 100%).The fluctuating pattern of appearance of ATI in patients with IBD was not associated with loss of clinical response or a rise in C-reactive protein. The authors suggest that in clinical decision making, only sustained appearance of ATI should be considered as an adverse therapeutic factor.