Outcomes of Infliximab-Treated inflammatory bowel disease patients undergoing therapeutic drug monitoring with two different assays

Objectives: There are multiple assays for infliximab (IFX) drug level (IFX-DL) and antibody to infliximab (ATI) measurement. The aims of this study are to examine the correlation and outcomes of IFX-DL and ATI in inflammatory bowel disease (IBD) patients, simultaneously measured with different methods in different institutions. Design and Methods: Residual samples of IFX-treated IBD patients undergoing drug monitoring for IFX-DL and ATI, both measured by ECLIA (Esoterix Laboratories) were used to simultaneously quantify IFX-DL via LC-MS/MS and ATI via an in-house ECLIA (ih-ECLIA) (Mayo Clinic Laboratories). Comparisons of IFX-DL and ATI detection between the assays from different institutions were performed, along with a comparison between the assays by association of IFX-DL and ATI obtained by each method with clinical remission, endoscopic healing (EH) and normal serum C-reactive protein (CRP ≤ 8 mg/L). Results: A total of 151 patients were included (median age, 32 years (range, 12–84); 45.7% female). The median IFX-DL was 7 mcg/mL (IQR: 1.3, 19.4) and 6 mcg/mL (IQR: 0.9, 20) via LC-MS/MS and ECLIA, respectively (Spearman correlation coefficient r = 0.97). ATI was detected in 13/142 (9.2%) via ih-ECLIA of whom 100% had IFX-DL < 5 mcg/mL by LC-MS/MS. ATI was positive in 39/151 (25.8%) via ECLIA, and 84.6% of positives had IFX-DL < 5 mcg/mL by ECLIA. Compared to ECLIA, the frequency of ATI detection via ih-ECLIA was lower in patients in clinical remission (7.3% vs 36.6%; p = 0.0005), those with normal CRP (5.9% vs. 20.0%; p = 0.0005), and in patients with EH (5.3% vs 18.4%; p = 0.03). Conclusions: IFX-DL was comparable between LC-MS/MS and ECLIA assays. Rate of ATI detection via ih-ECLIA was lower than ECLIA, which was more aligned with favorable clinical outcomes.