Association between kidney function and risk of hyponatremia: a single-center, cross-sectional study in the outpatient setting.

backgroundThe relationship between kidney function and hyponatremia risk remains poorly defined, particularly in patients with well-preserved kidney function. We aimed to clarify this association in an outpatient population across the full spectrum of kidney function.

methodsWe conducted a cross-sectional study of 130,194 outpatients in Japan between 2010 and 2020. Multivariable logistic regression assessed the association between estimated glomerular filtration rate (eGFR) and hyponatremia (serum sodium ≤ 135 mEq/L), adjusting for demographic, clinical, medication-related, and laboratory covariates. The relationship between eGFR and urine concentration in hyponatremic patients was also assessed using the urine-to-plasma osmolality ratio.

resultsThe prevalence of hyponatremia was 4.3%. Patients with hyponatremia were older and had more comorbidities. Hyponatremia was least common in the intermediate eGFR range (70-80 mL/min/1.73 m²) and increased toward lower and higher eGFR levels. This association persisted after adjustment for comorbidities and medications. Compared with the intermediate eGFR range, the association in the lower eGFR range was markedly attenuated after further adjustment for variables potentially related to vasopressin-mediated water regulation, whereas elevated odds persisted in the higher eGFR range. In the fully adjusted model, odds ratios (95% CI) were 4.63 (3.73-5.75) for eGFR ≥ 150 and 1.56 (1.02-2.39) for eGFR < 10 mL/min/1.73 m². Urine concentration was higher at higher eGFR levels than at lower eGFR levels.

conclusionsBoth lower and higher eGFR levels were associated with increased hyponatremia risk. Distinct patterns after covariate adjustment and differences in urine concentration suggest that the mechanisms underlying hyponatremia differ between lower and higher levels of kidney function.