Higher 24-Hour Urine Sodium-to-Potassium Ratio Is Associated With Worse Renal Outcome, Independent of Hypertension

Objective: Low sodium and high potassium intake are advised to control blood pressure and may improve long term cardiorenal outcomes. The optimal potassium intake for patients with chronic kidney disease remains to be defined. We investigated whether the 24-hour urine sodium-to-potassium ratio (Na/K-ratio), which incorporates estimates of both sodium and potassium intake, could predict long-term renal outcome. Design and method: We selected adult outpatients of a Dutch tertiary hospital who collected at least one 24-hour urine collection with available sodium and potassium measurements between 1998-2023. Patients with a history of dialysis or kidney transplantation were excluded. We examined the need for persistent renal replacement therapy (RRT), defined as dialysis or kidney transplantation, using Cox regression. The association between the 24-hour urine Na/K-ratio and the estimated glomerular filtration rate (eGFR) slope was assessed using linear regression in patients who had >2 eGFR measurements available in >2 years. The linear regression analysis was adjusted for age, sex, baseline eGFR, diabetes mellitus and hypertension. In the Cox regression analysis, we additionally adjusted for the interaction between the 24-hour urine Na/K-ratio and baseline eGFR. Hypertension was defined as a clinical diagnosis or an office blood pressure >139/89 mmHg. Results: We included 720 patients aged 50±15 years, of whom 52% were male, 58% had hypertension and 31% diabetes mellitus. The baseline eGFR was 64±34 ml/min/1.73m2. The 24-hour urine Na/K ratio was 2.5±1.2 in a median number of 1 (IQR=1-2) urine collection. During a median follow-up of 11.2 (14.0) years, 29% of patients needed RRT. In 575 patients with sufficient eGFR measurements (mean 12±8 measurements per subject) the median annual eGFR decline was 1.51(IQR=0.49-3.98) ml/min/1.73m2. Every unit increase in the 24-hour urine Na/K-ratio was associated with a 19% higher risk for persistent RRT need (HR 1.19, 95% CI 1.03-1.39; p=0.02) and an additional 0.44 ml/min/1.73m2 (95% CI -0.83, -0.04; p=0.03) decline in eGFR. Conclusions: This study suggests that higher 24-hour urine Na/K-ratio is associated with faster kidney function decline and the need for RRT, independent of hypertension, in a selected group of outpatients.