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Objective To characterise the temporal evolution of antibodies to infliximab (ATI). Design Prospective observational study of infliximabtreated patients with inflammatory bowel disease between 2009 and 2012. Interventions Trough levels of infliximab and ATI were measured before each infusion by anti-X ELISA. Patients were monitored for disease activity by clinical activity indexes and for dose-intensification or infliximab cessation. The occurrence of transient ATI disappearing spontaneously without intervention was recorded separately. Results 125 patients were included (98 Crohn's disease, 27 ulcerative colitis, median follow-up 11.5 ±22 months) and 1119 sera were analysed for infliximab and ATI levels. Kaplan-Meier analysis showed that 42% of patients remained ATI-free by 4 years of treatment. Most (90%) of the patients who developed ATI did so within the first 12 months of therapy, whereas transient ATI were detected throughout the duration of infliximab therapy (p<0.001). ATI incidence was similar between patients who received infliximab previously (episodic/ interrupted therapy patients, n=14) and scheduledtherapy patients (n=111). In the scheduled group, combination immunomodulator+infliximab resulted in longer ATI-free survival compared with monotherapy (p=0.003, logrank test). Survival free of clinical loss of response was enjoyed by 51% of patients, and serial measurements showed that ATI development often preceded the onset of clinical flare. Conclusions When followed prospectively, most patients who develop ATI do so within the first 12 months of therapy. This incidence is reduced by concomitant immunomodulator even in scheduledtherapy patients. In contrast, transient ATI, which are of little clinical significance, can appear haphazardly at any time during treatment. The onset of clinical loss of response may lag behind the appearance of antiinfliximab antibodies. [ABSTRACT FROM AUTHOR]