Several techniques are used to measure infliximab (IFX) and anti-IFX antibodies (Abs) in Crohn's disease. The aim of this study was to compare different assays for this purpose.
Fluid-phase radioimmunoassay (RIA), solid-phase enzyme-linked immunosorbent assay (ELISA), reporter gene assay (RGA), and enzyme immunoassay (EIA; anti-IFX Ab only) were assessed. IFX was added to pooled serum from 13 patients with inactive Crohn's disease to yield concentrations of 0, 1, 3, and 9 µg/mL. Anti-IFX Abs were assessed in 6 patients.
IFX assessments: RIA and RGA had lower limit of detection than ELISA (0.07 µg/mL and 0.13 versus 0.26). Maximal inaccuracies were 39%, 24%, and 23%. Imprecisions (coefficients of variation) were ≤20% within IFX concentrations between 1 and 9 µg/mL. All assays showed linear correlations (R2 = 0.97–0.99), but sample concentrations differed by up to 1.55 µg/mL for RIA and RGA, 1.41 µg/mL for ELISA and RIA, and 0.48 µg/mL for ELISA and RGA (P < 0.05). Anti-IFX Ab assessments: RGA gave highly reproducible results (coefficients of variation ≤ 7%) compared with all others (24%–26%). All assays had linear correlations (R2 = 0.71–0.93), except ELISA versus RGA and EIA. Assays disagreed on anti-IFX Ab titers with mean difference −420 (−1200 to 210) in RGA and EIA, and up to 4500 (−2700 to 11,800) in RIA and RGA. A contributing factor to these discrepancies was inability of ELISA to detect IgG4 anti-IFX Abs.
Performances of assays for IFX and anti-IFX Abs are comparable. However, IFX concentrations and anti-IFX Ab titers show systematic differences, and in individual patients, only the same assay should be used. Problems may arise when different assays are used to manage therapies in the same patient.