Rituximab pharmacokinetics in patients with membranous nephropathy (MN) exhibit significant interindividual variability. Accurate measurement of serum rituximab concentrations is essential for effective therapeutic monitoring. This study develops a highly sensitive time-resolved fluoroimmunoassay (TRFIA) for rituximab (rituximab-TRFIA) with a wide detection range, aimed at enhancing therapeutic drug monitoring in MN treatment. A capture -type rituximab-TRFIA was developed using streptavidin-coated microplates, biotinylated anti-rituximab idiotypic antibodies, and Eu3+-labeled mouse anti-human IgG targeting the Fc fragment of rituximab. The assay was used to measure rituximab serum concentrations in MN patients treated with rituximab. The linear range of rituximab-TRFIA is 0.50 to 2500ng/mL, with a limit of detection (LOD) of 0.062ng/mL. The intra-assay coefficient of variation (CV) ranges from 1.97% to 9.50%, while the inter-assay CV ranges from 7.44% to 9.99%. The recovery rate ranges from 99.14% to 107.75%. No cross-reactivity was observed with other monoclonal antibody drugs (mAbs). The detection range is two orders of magnitude higher, and the sensitivity is six times greater than that of the enzyme-linked immunosorbent assay (ELISA). Rituximab-TRFIA showed strong consistency within the same measurement range compared to ELISA (P < 0.0001). The serum rituximab levels in MN patients were significantly higher than those in the control group (P < 0.0001). Among patients who did not achieve remission, 60% to 70% had insufficient drug concentrations, and rituximab pharmacokinetics followed the expected trends. The rituximab-TRFIA provides high sensitivity, a wide detection range, and reliable performance for monitoring serum rituximab concentrations in MN patients, supporting personalized treatment strategies.
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