Comparative study of a point-of-care test and an enzyme-linked immunosorbent assay (ELISA) for infliximab levels

Background: Proactive therapeutic drug monitoring (TDM) is often challenged by long turnaround time when using enzyme-linked immunosorbent assays (ELISAs), especially when analyses are centralised. Point-of-care tests (POCTs) allow rapid assessments, but data on their agreement with existing in-house methodologies are scarce.Objective: To examine the agreement between a POCT by ProciseDx (San Diego, CA) and the most frequently used in-house ELISA for infliximab (IFX) quantification in Sweden.Methods: Serum samples were analysed using the in-house ELISA, Karolinska University Hospital, Stockholm, Sweden and a POCT by ProciseDx (San Diego, CA). Agreement was assessed and differences were examined.Results: Samples from 61 inflammatory bowel disease (IBD) patients were analysed with a median IFX concentration of 7.9 mu g/mL (interquartile range (IQR) 5.5-13) for the POCT and 7.9 mu g/mL (IQR 5.2-12) for the ELISA (Pearson's correlation coefficient = 0.95 (95% CI 0.92-0.97, p < .01)). A Passing-Bablok regression yielded an intercept of -0.44 and a slope of 1.09. The Bland-Altman plot showed a systemic bias of -0.77 mu g/mL (95% CI -0.18 to -1.4) between the methods. The upper limit of agreement was 3.7 (95% CI 2.7-4.8) (mu g/mL), whereas the lower limit agreement was -5.3 (95% CI -6.3 to -4.3) (mu g/mL). An excellent reliability was observed, intraclass correlation showed = 0.94 (95% CI 0.89-0.96, p < .0001). When defining IFX concentration as subtherapeutic (<3.0 mu g/mL), therapeutic (3.0-7.0 mu g/mL) or supratherapeutic (>7.0 mu g/mL) drug levels, Kappa statistics showed a substantial agreement (0.79).Conclusions: The POCT by ProciseDx (San Diego, CA) demonstrated a good agreement with the in-house ELISA, supporting its use for rapid IFX quantification.