Dietary intake greatly influences the urinary excretion of sodium. The rate of sodium excretion during the night is only onefifth of the peak rate during the day, indicating a large diurnal variation. Measurement of urinary sodium is helpful in the differential diagnosis of hyponatremia and hypernatremia.
Urine Sodium Concentration in Patients with Hyponatremia
|
Fluid Volume |
Urinary Sodium |
Causes |
|
Hypovolemia |
>20 |
Renal losses due to diuretics, aldosterone deficiency, salt losing nephropathy, osmotic diuresis, ketonuria, RTA |
|
|
<20 |
Extra renal losses due to vomiting, diarrhea, third spacing of fluids from burns, pancreatitis, trauma |
|
Euvolemia |
>20 |
Cortisol deficiency, hypothyroidism, stress, drugs, SIADH |
|
Hypervolemia |
>20 |
Acute or chronic renal failure |
|
|
<20 |
Nephrotic syndrome, cirrhosis, cardiac failure |
Urine Sodium Concentration in Patients with Hypernatremia
|
Fluid Volume |
Urinary Sodium |
Causes |
|
Hypovolemia |
<20 |
Excess sweating, burns, diarrhea, fistulas |
|
|
>20 |
Renal disease, urinary tract obstruction, osmotic or loop diuretics |
|
Euvolemia |
Variable |
Diabetes insipidus, hypodipsia, Insensible losses, respiratory, dermal |
|
Hypervolemia |
>20 |
Primary hyperaldosteronism, Cushing’s syndrome, hypertonic dialysis, hypertonic sodium bicarbonate, sodium chloride tablets |
Urine sodium concentration is measured using an ion-selective electrode. Reference range is 20-320 mEq/L.
Specimen requirement is a 24 hour urine collection in a container without preservative. Specimen should be refrigerated within four hours of completion of the collection.
References
Harrington JT, Cohen JJ. Measurement of urinary electrolytes—indications and limitations. N Engl J Med. 1975; 293(24):1241-1243.
Sherman RA, Eisinger RP. The use (and misuse) of urinary sodium and chloride measurements. JAMA. 1982; 247(22):3121-3124
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