Agreement in assessment of infliximab and adalimumab levels in rheumatoid arthritis: interlaboratory and interassay comparison

Objective Infliximab (IFX) and adalimumab (ADL) drug levels and anti-drug antibodies (ADA) are assessed using a variety of techniques, therefore, results cannot accurately be compared for clinical purposes. The aim of this study was to test two infliximab (IFX) and adalimumab (ADL) ELISA versions, for drug levels and ADA, to see whether they yield similar results. Methods ELISA versions [Promonitor (R) IFX R1 and R2 (V1), Promonitor (R) IFX and Anti-IFX (V.2); Promonitor (R) ADL RI and R2 (V.1), Promonitor (R) ADL and Anti-ADL (V.2) kits (Progenika Biopharma, Spain)] were used to measure drug levels and ADA in IFX (n=24) and ADL (n=24) rheumatoid arthritis-treated patients in three independent laboratories. Quantitative and qualitative agreements were evaluated using intraclass correlation coefficients (ICC), and Cohen's Kappa (K) respectively. The Bland-Altman plots assessed differences between V.1 and V.2. Results Interlaboratory agreement (ICCIx).with V.1 was poor for IFX (0.66/0.62) and ADL (0.69/0.52) drug levels; meanwhile, high agreement was found with V.2 for IFX (0.98/0.95) and ADL (0.094/1.00). Comparison between V.1 and V.2 in each laboratory resulted in systematically higher values in V.2 than in V.1 and poor agreement (ICC/kappa ranges) for IFX - (0.12-0.71 0.19-0.42) and ADL (0.69-0.89/0.50-0.73). Conclusion Qualitative measurements result in better agreement, as evidenced in our study. Greater agreement in V.2 compared with V.1 for IFX and ADL levels could be due to a better tune up. Further studies are required to standardise methods to establish therapeutic reference ranges.