VARIABILITY OF 24-HOUR URINE SODIUM EXCRETION AND LONG-TERM CARDIOVASCULAR AND RENAL OUTCOME

OBJECTIVE: High sodium intake is an important risk factor for cardiovascular disease, and is often monitored by measuring 24-hour urine sodium excretion. Besides sodium intake, the variability in 24-hour urine sodium excretion has been shown to be mediated by infradian aldosterone and cortisol rhythms. Considering the detrimental cardiovascular and renal effects of aldosterone excess, one may hypothesize that 24-hour urine sodium excretion variability may be associated with long-term cardiovascular and renal disease. DESIGN AND METHOD: In a retrospective cohort study, we included adult subjects with an estimated glomerular filtration rate (eGFR) > 60 ml/min/1.73 m2 who collected an outpatient 24-hour urine sample between 1998 and 2000, and had collected at least 6 additional 24-hour urine collections during follow-up. We quantified the proportional variability among 7 consecutive 24-hour sodium excretion measurements by calculating the coefficient of variation (CV). We assessed the association between 24-hour sodium excretion variability and cardiovascular (coronary, cerebrovascular event, ospitalised heart failure or mortality) and renal (60% eGFR reduction, dialysis, kidney transplantation or mortality) outcome, using Cox regression analyses with correction for traditional cardiorenal risk factors. RESULTS: We included 316 subjects with an average age of 44 years and a mean eGFR of 90 ml/min/1.73 m2. 46% were male, 50% had hypertension and 30% had diabetes. Median follow-up was 16.4 years during which 85 cardiovascular and 131 renal endpoints occurred. The CV for 24-hour urine sodium excretion was 8.3% (SD 1.6). Subjects with a high variability in 24-hour sodium excretion were more often female, had less often diabetes and a lower 24-hour urine sodium-to-potassium ratio. High 24-hour urine sodium excretion variability was associated with worse long-term renal outcome (p = 0.03), which was observed in the female subgroup (p = 0.02) but not in males (p = 0.65) (Figure). No linear association was found for cardiovascular outcome (p = 0.89), but the middle tertile was associated with an increased risk (OR 2.29, 95% CI 1.24-4.25). CONCLUSIONS: High 24-hour urine sodium excretion variability is associated with an increased risk for long-term renal disease.