Infliximab (IFX) is a monoclonal antibody used to treat patients with inflammatory bowel disease (IBD). For IFX therapeutic drug monitoring (TDM), the most commonly used analysis is enzyme-linked immunosorbent assays (ELISA) which do not allow results to be provided in real-time. The aim of this study was to compare the in-house ELISA (Promonitor IFX) with the much faster assay Quantum Blue® IFX (QB) for quantification of serum IFX concentration among IBD patients in maintenance IFX therapy.
We studied 30 serum samples from outpatients in IFX maintenance therapy at Copenhagen University Hospital Hvidovre, Denmark. Samples were used to compare IFX measurements from Promonitor IFX with QB. Therapeutic intervals of <3 μg/mL, 3-7 μg/mL and >7 μg/mL were equally covered. Differences were evaluated using Bland-Altman plots and Student t-test. Correlation was evaluated using x,y-plot and Pearson's correlation coefficient. The intermediate imprecision (CV%) of QB was measured at two levels (3 μg/mL and 7 μg/mL). For qualitative comparison, weighted kappa statistics (κ) were determined after stratification of results by therapeutic interval.
Promonitor IFX and QB were strongly correlated (r = 0.92, p < 0.001). The mean difference between Promonitor IFX and QB was -0.57 μg/mL (p = 0.2). The CV% of QB was 16.3% at 3 μg/mL and 16.7% at 7 μg/mL. Classification of results according to therapeutic interval showed almost perfect agreement (κ = 0.81).
QB is a suitable alternative to Promonitor IFX for TDM in patients treated with IFX for IBD. The results revealed a strong correlation between methods, in particular at lower IFX concentrations, representing the most interesting clinical range. When the samples were stratified according to the therapeutic interval, an almost perfect agreement between the methods was observed.