Background: The aim of this study was to evaluate reliability of four different assays for measuring infliximab trough levels and antibodies to infliximab (ATI).
Methods: In this non-interventional, cross-sectional study including IBD patients, infliximab levels and ATI were measured using four different assays: Lisa-Tracker, Promonitor, Q-Inflixi and Sanquin. Reliability and agreement for infliximab levels was assessed using the intraclass correlation coefficient (ICC) and Bland-Altman plots. Qualitative agreement for infliximab (based on a pre-established target window of trough levels between 3 µg/ml and 7 µg/ml) and for ATI were estimated by Cohen's kappa.
Results: Serum samples of 84 IBD patients were evaluated for infliximab using the four assays. Reliability was 'substantial' between Lisa-Tracker versus Promonitor and 'almost perfect' between the remaining assay pairs, with ICCs [95% confidence interval (CI)] ranging from 0.93 (0.70-0.97) for Lisa-Tracker versus Promonitor to 0.97 (0.95-0.98) for Q-Inflixi versus Sanquin. Bland-Altman plots showed significant bias between assays except Promonitor versus Q-Inflixi, which had excellent agreement. The greatest differences in mean infliximab were found between Promonitor versus Lisa-Tracker (-0.91 µg/ml) and Lisa-Tracker versus Q-Inflixi (0.69 µg/ml). Qualitative agreement for infliximab was 'almost perfect' for Promonitor versus Q-Inflixi (kappa 0.84) and Q-Inflixi versus Sanquin (kappa 0.81), and 'substantial' for the remaining pairs. More than 10% of patients who had infliximab levels within the target interval by Lisa-Tracker had suboptimal concentrations (<3 µg/ml) with Promonitor and Q-Inflixi. Furthermore, 11% of patients within the target interval by Q-Inflixi had supra-optimal levels (>7 µg/ml) by Lisa-Tracker. In the remaining paired comparisons, fewer than 5% of patients were placed in different subgroups. Qualitative agreement for ATI fluctuated between 'moderate' and 'almost perfect'.
Conclusions: All four assays seem suitable for therapeutic drug monitoring of infliximab. Promonitor and Q-Inflixi had the best agreement, making those assays fully interchangeable. Systematic biases between Lisa-Tracker with Promonitor and Q-Inflixi suggest that these assays should not be interchanged during the follow up of an individual patient.
Keywords: antibodies to infliximab; inflammatory bowel disease; infliximab; intraclass correlation coefficient; reliability; therapeutic drug monitoring; trough levels.