Clinical relevance and inter-test reliability of anti-infliximab antibodies and infliximab trough levels in patients with inflammatory bowel disease.

Treatment with infliximab is a common option for inflammatory bowel disease (IBD) patients. Therapeutic drug monitoring could improve treatment management.To test inter-test reliability of two commercially available diagnostic kits for infliximab trough levels and infliximab antibodies, and their association with treatment outcomes.86 IBD outpatients on infliximab maintenance treatment were enrolled in a prospective cross-sectional study, 115 samples were available for inter-test reliability.Inter-test agreement was good both for trough levels (concordance correlation coefficient 0.78, weighted κ 0.60, Sperman's ρ 0.937) and for infliximab antibodies (weighted κ 0.79) measurement, when comparing Promonitor and ImmunDiagnostik kits. According to manufacturers' cut-off values, trough levels were classified as undetectable (17%), low (21%) or in range (63%). The only significant associations were: mucosal healing (p=0.026; OR 6.50), infliximab antibody status (p=0.0015; OR 0.031) and adverse events (p=0.009; OR 0.115). Higher trough levels were observed among patients on concomitant steroid/immunosuppressive therapy and among patients with dose-intensification. Infliximab antibodies were significantly associated to treatment-related adverse events (p=0.0003, OR 30.42), and to lower trough levels, but not to other clinical variables.The two tests performed equally well. Infliximab antibodies were associated to adverse events, while trough levels were not associated to treatment outcomes.