Normal Urine Sodium Levels
In individuals with normal kidney function and no underlying medical conditions, normal urine sodium concentration typically ranges from 40-220 mEq/L (or mmol/L), with values varying significantly based on dietary sodium intake.
Understanding Urine Sodium Variability
Urine sodium is not a fixed value but rather reflects dietary intake and kidney function. Unlike serum sodium (which is tightly regulated at 135-145 mEq/L), urine sodium fluctuates widely based on what you eat 1, 2.
Key Physiological Principles
- Dietary correlation: In healthy individuals, urine sodium excretion matches dietary sodium intake over 24 hours, typically ranging from 40-220 mEq/L depending on salt consumption 3, 2
- Time-of-day variation: Spot urine sodium values are significantly higher in evening samples compared to morning samples, even in healthy individuals 3
- Kidney function marker: Urine sodium >140 mEq/L (above normal plasma concentration) serves as a biomarker of normal or improving renal function in critically ill patients 4
Clinical Context for Interpretation
Normal Kidney Function
- Spot urine sodium: 40-220 mEq/L is typical, with most healthy adults on Western diets having values of 80-150 mEq/L 3, 2
- 24-hour urine sodium: Should approximate dietary intake; for a typical 2g sodium/day diet, this equals approximately 87 mEq/24 hours 3
- Functional marker: Values >140 mEq/L indicate robust natriuretic capacity and normal glomerular filtration 4
Relationship to Dietary Sodium Intake
For adults with normal kidney function, recommended sodium intake is <2.3g/day (100 mmol/day), which translates to a mean spot urine sodium of approximately 83 mEq/L 5, 3. Higher dietary intake produces proportionally higher urine sodium values 3.
Important Clinical Caveats
Do not confuse urine sodium with serum sodium—these are completely different measurements. Serum sodium reflects body water balance (normal 135-145 mEq/L), while urine sodium reflects dietary intake and kidney handling of sodium 1, 2.
When Urine Sodium Interpretation Changes
- Acute kidney injury: Urine sodium <20 mEq/L suggests prerenal azotemia (reversible), while >40 mEq/L suggests acute tubular necrosis 6
- Hyponatremia evaluation: Urine sodium <30 mEq/L indicates appropriate renal sodium conservation (hypovolemia), while >40 mEq/L suggests SIADH or salt-wasting 1, 2, 7
- CKD patients: Urine sodium progressively decreases with advancing kidney disease, with lower values in stage IV-V CKD compared to earlier stages 3
Medications Affecting Urine Sodium
Diuretics artificially elevate urine sodium (often >100 mEq/L) regardless of volume status, making interpretation unreliable within 24-48 hours of administration 1, 2. High-sodium medications like piperacillin/tazobactam (162 mg sodium per 3.375g dose) can also increase urine sodium excretion 8.
Practical Takeaway
For a healthy adult with normal kidney function eating a typical Western diet, expect spot urine sodium values between 80-150 mEq/L 3, 2. Values significantly below 40 mEq/L suggest either very low dietary sodium intake or impaired renal function, while values consistently >170 mEq/L indicate either very high dietary intake or enhanced natriuresis 4, 3.