Association between Urinary Fractional Excretion of Potassium and Proteinuria Remission in Adult Nephrotic Syndrome

Key Points. Elevated fractional excretion of potassium (FEK) at biopsy predicts a lower rate of proteinuria remission in adults with nephrotic syndrome. FEK levels vary by histologic types, being low in minimal change disease and high in diabetic kidney disease, suggesting diagnostic utility. FEK shows a dose-dependent link with remission, supporting its use as a simple prognostic biomarker. Background. Reliable prognostic biomarkers of nephrotic syndrome (NS) are crucial for optimizing patient management. The fractional excretion of potassium (FEK) reflects net tubular potassium handling, but its association with prognosis in NS remains unexplored. We aimed to investigate the association between baseline FEK and proteinuria remission in adult NS. Methods. This multicenter retrospective cohort study was conducted across nine institutions in Japan. We enrolled adult patients diagnosed with NS who underwent a native kidney biopsy and had FEK between January 2012 and June 2022. Patients were categorized based on FEK levels and followed until the first complete remission of proteinuria. Results. A total of 401 patients (median age, 61 years; 43% female) were included. Study participants were stratified into two groups based on the FEK cutoff value of 10%. Patients with high FEK had a significantly lower cumulative incidence of complete remission compared with those with low FEK (P < 0.001, log-rank test). In multivariable Cox regression analysis, high FEK was independently associated with a lower likelihood of complete remission (hazard ratio, 0.53; 95% confidence interval, 0.36 to 0.78). The trend of this association was observed across most subgroups, including those based on histologic diagnosis of minimal change disease and non–minimal change disease. Further stratification into four FEK categories revealed a progressive decline in remission rates with increasing FEK, indicating a dose-dependent relationship. Conclusions. Elevated FEK is an independent predictor of a lower likelihood of proteinuria remission in Japanese adults with NS. FEK may serve as a readily accessible and valuable prognostic biomarker in the clinical management of NS.