Detection of anti-drug antibodies using a bridging ELISA compared with radioimmunoassay in adalimumab-treated rheumatoid arthritis patients with random drug levels

Objective: To determine the concordance between RIA and bridging ELISA at detecting anti-drug antibodies (ADAbs) in the context of random adalimumab levels and investigate the additional clinical utility of detecting ADAbs in RA patients who test ADAb positive by RIA and negative by ELISA. Methods: ADAb levels were determined using RIA and bridging ELISA in 63 adalimumab treated RA patients (159 samples). Immunogenicity concordance was determined using receiver-operating characteristic (ROC) curves. To determine the additional clinical value provided by a positive RIA in the presence of negative ELISA, association between treatment response (ΔDAS28), adalimumab drug levels and ADAbs was evaluated longitudinally using generalised estimating equation. Results: Of the 60 RIA+ samples (n=31 patients), 19 (n=10 patients) were also ELISA+, corresponding to 31.7% of samples. Area under the curve (AUC) for detecting ADAbs using ELISA (compared with RIA) using ROC curves was 0.65 (95% CI: 0.59-0.71); this increased to 0.91 (95% CI: 0.81-0.99) if ADAbs were ≥100 AU/ml using RIA. In RIA+/ELISA- patients, adalimumab levels were associated with ΔDAS28 over 12 months [regression coefficient: 0.098 (0.043-0.15), p