Cut-off ranges of infliximab serum levels in Crohn's disease in the clinical practice.

Introduction: between 30 % and 40 % of patients treated with infliximab lose response during maintenance. Therapeutic drug monitoring could be used to optimize management in these situations. However, infliximab serum levels are not well defined. The aim of this study was to determine the cut-off range of infliximab serum levels in Crohn's disease patients in remission in the clinical practice.

Methods: an observational retrospective study was performed from 2016 to 2017. Patients were included with established Crohn's disease, who had been on a maintenance dose schedule of infliximab. Infliximab levels and antibodies to infliximab were measured at least twice in all patients, after induction and after six months of treatment. Clinical remission was defined as ≤ 4 using the Harvey-Bradshaw index. Cluster analysis was used to analyze the results.

Results: one hundred and five Crohn's disease patients were included in the study; 57.1 % were male with a mean age of 39 years (SD ± 12.9). The median (range) time of the disease was eleven years (7-15) and the median (range) time of follow-up was 32 months (22-38). Patients who achieved remission had infliximab serum levels between 4.26-8.26 ug/ml versus 0.06-1.43 ug/ml in patients who did not achieve remission after induction. Infliximab serum levels were 2.84-7.75 ug/ml and 0.05-2.69 ug/ml in patients who achieved remission versus those who did not achieve remission after six months of treatment. Overall, 4.26-8.26 ug/ml was found to be the best cut-off range for remission.

Conclusions: in our clinical practice, serum levels of infliximab in Crohn's disease patients should be higher than 4 ug/ml to achieve clinical remission.