Drug concentrations and anti-drug antibodies in patients with arthritis treated with infliximab and adalimumab - comparison of three assays

This study aimed to compare three different assays for assessment of circulating concentrations of infliximab, adalimumab and anti-drug antibodies (ADAs) in patients with inflammatory arthritis. Concentration of drug and ADAs were determined by immunoassays from IDKmonitor and Promonitor, and an in-house bead-based assay in plasma samples from patients with rheumatoid arthritis, psoriatic arthritis or axial spondyloarthritis treated with infliximab ( n = 90), adalimumab ( n = 90), or bio-naïve ( n = 20). Correlations of absolute and ranked drug levels were analyzed. Ranking of drug concentrations determined by the three assays correlated well, but agreement with respect to absolute drug levels was moderate (Cohen´s κ: 0.60–0.82 for infliximab and 0.15–0.42 for adalimumab). The Promonitor and the bead-based assay agreed, although not completely, with respect to identifying ADA-positive samples. In accordance with its ability to detect ADA in the presence of drugs, the IDKmonitor assay detected more ADA-positive samples (anti-infliximab n = 50 vs. 13 and 10, respectively, anti-adalimumab n = 26 vs. 12 and 10) capturing ADA-positive samples determined by the other assays, except one. The assays correlated acceptably with respect to relative drug levels but diverged regarding the absolute concentration. Thus, use of the same assay is strongly recommended for longitudinal assessments.