Sodium - urine

Last updated: Saturday, 27, March, 2004

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Item Process
Specimen

Random or 24 hour urine collection.

Method

ISE or flame emission spectrophotometry.

Reference Interval

In hyponatraemia or hypovolaemic shock without acute tubular necrosis, urine sodium should be <20 mmol/L and fractional excretion of sodium should be <1.5%.

If extracellular fluid volume and plasma sodium are normal, urine sodium should equal intake minus non-renal losses, typically 75-300 mmol/24 hours.

Application

Investigation of hyponatraemia.

Assessment of renal function in hypovolaemic shock.

Investigation of compliance with a low sodium diet.

Investigation of predisposing factors for hypercalciuria in patients with renal calculi.

Interpretation

Urinary sodium excretion exceeds 20 mmol/L in hyponatraemia due to SIADH, diuretic therapy, or Addison’s disease.

In a patient with shock and oliguria a urinary sodium >20 mmol/L or a fractional excretion of sodium >1.5% suggests acute tubular necrosis.

High urine sodium increases urine calcium and predisposes to calculi containing calcium.

Reference

Halperin ML and Bohn D. Crit Care Clin. 2002; 18(2): 249-272.