Objectives Therapeutic drug monitoring of infliximab (IFX) is important to optimise treatment of inflammatory bowel disease (IBD). A recent IBD consensus statement recommends targeting trough serum concentrations of >3 μg/mL, higher than our local recommendation of >1 μg/mL. We therefore investigated the relationship between IFX trough concentrations and C reactive protein (CRP), faecal calprotectin (FCP), clinical outcomes and anti-IFX antibody (AB) development as well as the influence of concomitant thiopurine treatment.

Methods Observational data, prospectively collected in a cohort of adult patients with IBD newly initiated on IFX at a single centre.

Results IFX concentrations >3 μg/mL were associated with a greater reduction in CRP (% change from baseline) and lower FCP; mean (SD) 47 (33.8) % vs 102.3 (136.9) % and 233.9 (505.1) μg/g vs 416.3 (613.5) μg/g, respectively. Lower IFX concentrations were observed in patients who developed AB than those who did not, mean (range) 6.2 (1.1–10) μg/mL vs 0.9 (0.4–4.9) μg/mL, respectively, and also in patients who stopped/switched therapy compared with those who continued, 2.4 (2.9) μg/mL vs 6.5 (2.8) μg/mL; p=0.0002. Patients taking a concomitant thiopurine were found to have higher IFX concentrations; mean (range) 6.4 (0.7–10) μg/mL vs 3.9 (0.4–10) μg/mL.

Conclusions IFX concentrations are correlated with biomarkers, clinical response and AB development in patients with IBD. Concomitant thiopurine therapy appears to be associated with higher IFX concentrations and reduced likelihood of AB development.