Comparison data regarding anti-tumor necrosis factor drug concentrations in inflammatory bowel disease (IBD) between the enzyme-linked immunosorbent assay (ELISA) and the homogenous mobility shift assay (HMSA) are scarce.1-3 As decisions in clinical practice depend on the thresholds that define a therapeutic drug concentration, it is important to determine if this varies based on the type of assay used for therapeutic drug monitoring.4 We recently showed a discrepancy between a commercially available ELISA and the HMSA for both infliximab and adalimumab concentrations in patients with IBD.5 Based on the results of the study, Prometheus Laboratories (San Diego, CA) initiated a comprehensive review of their HMSA assays and found that there was an upward drift for both infliximab (from December 2017 to May 2019) and adalimumab (from August 2017 to May 2019), including when our study was performed. Prometheus Laboratories corrected the errant values and reported the revised drug concentrations to physicians (Supplementary Methods). We aimed to compare the corrected infliximab and adalimumab concentrations with the original ELISA values.
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