Baseline fractional excretion of IgG predicts clinical response to rituximab in primary membranous nephropathy: a retrospective cohort study

Reliable biomarkers for predicting response to rituximab in primary membranous nephropathy (PMN) are limited. Whether fractional excretion of IgG (FEIgG), as a quantitative indicator of glomerular size-selective dysfunction and podocyte injury, provides predictive value in patients receiving rituximab therapy remains unclear. This study investigated whether baseline fractional excretion of immunoglobulin G (FEIgG) predicts clinical remission after rituximab therapy and provides additional information beyond anti-phospholipase A2 receptor (anti-PLA2R) antibody. This retrospective study included 117 patients with biopsy-proven PMN who received rituximab and completed at least 6 months of follow-up. The association between baseline FEIgG and clinical remission was evaluated using multivariable logistic regression. Predictive performance and incremental value beyond anti-PLA2R antibody were assessed using receiver operating characteristic (ROC) analysis, calibration, reclassification, and decision curve analyses. At 6 months, 23 (19.7%) patients achieved complete remission, 51 (43.6%) achieved partial remission, and 43 (36.8%) remained in non-remission status. Baseline FEIgG increased progressively from complete remission to partial remission and non-remission groups (P for trend < 0.05). After adjustment for demographic characteristics, renal parameters, anti-PLA2R antibody levels, and treatment-related factors, higher baseline FEIgG remained independently associated with a lower likelihood of clinical remission (OR = 0.808, 95% CI 0.668–0.978, P = 0.028). FEIgG alone showed moderate predictive performance for remission (AUC = 0.730, 95% CI 0.638–0.822). Adding FEIgG to anti-PLA2R antibody increased the AUC from 0.671 to 0.711, although the improvement was not statistically significant (DeLong test, P = 0.382). The combined model demonstrated improved prediction accuracy (Brier score: 0.205 vs. 0.222), risk reclassification (NRI = 0.463; IDI = 0.071), and potential clinical utility. Baseline FEIgG is an independent predictor of clinical remission after rituximab therapy in PMN. FEIgG provides complementary prognostic information beyond anti-PLA2R antibody and may improve individualized prediction of treatment response.