Normal Urine Spot Sodium Values
A normal spot urine sodium concentration is typically between 20-40 mmol/L, with values >50-70 mmol/L indicating adequate natriuresis after diuretic administration and values <20 mmol/L suggesting sodium retention. 1
Clinical Significance of Spot Urine Sodium Values
- Low urine sodium (<20 mmol/L) suggests sodium retention, which may occur in volume depletion, hepatorenal syndrome, and prerenal azotemia 1
- Moderate urine sodium (20-40 mmol/L) may be seen in recent diuretic use, partial volume depletion, and early acute kidney injury 1
- High urine sodium (>140 mmol/L) typically indicates normal renal function, even in patients receiving diuretics 2
Spot Urine Sodium in Specific Clinical Contexts
Heart Failure
- In acute heart failure, a spot urine sodium concentration <50-70 mEq/L at 2 hours after loop diuretic administration indicates insufficient diuretic response 3
- Higher spot urine sodium levels (≥65 mmol/L) after diuretic administration identify heart failure patients likely to respond to ambulatory diuretic infusion with lower rates of hospitalization 4
- Spot urine sodium measurement 2 hours after the first diuretic dose reliably predicts subsequent 6-hour natriuresis 5
Cirrhosis with Ascites
- In cirrhotic patients with ascites, a spot urine sodium/potassium ratio >1 correlates with 24-hour sodium excretion >78 mmol/day with approximately 90% accuracy, indicating adequate natriuresis 5, 1
- A spot urine sodium/potassium ratio between 1.8 and 2.5 has a sensitivity of 87.5% in predicting a 24-hour urinary sodium excretion of 78 mmol/day 3
- For diagnosis of hepatorenal syndrome, urine sodium <10 mEq/L is a supportive finding 1
SIADH
- In the diagnosis of SIADH, urine sodium >20 mEq/L despite hyponatremia is considered inappropriate renal sodium wasting 1, 6
- The American Thoracic Society suggests that urine sodium of 77 mEq/L (>20 mEq/L) indicates inappropriate renal sodium wasting despite presumed hyponatremia 6
Prognostic Value
- In critically ill patients, a urine sodium concentration greater than 140 mmol/L (equivalent to normal plasma sodium) is associated with normal renal function and better outcomes 2
- In heart failure patients, spot urine sodium levels >60 mmol/L measured at day 3 of hospitalization are associated with improved mid-term outcomes 7
- In patients with chronic kidney disease, spot urine sodium measurements can accurately predict 24-hour sodium excretion, especially in those with eGFR <30 mL/min 8
Clinical Pitfalls to Avoid
- Not accounting for the effects of diuretics, which can artificially increase urine sodium excretion despite volume depletion 1
- Failing to consider that patients who have recently received diuretics may have higher urine sodium levels despite having conditions that would normally cause sodium retention 1
- Using spot urine sodium alone without considering clinical context, as values must be interpreted differently in heart failure, cirrhosis, and other conditions 5, 1