When may the urine sodium concentration be misleading?
The urine sodium concentration may be misleading in the following circumstances.
• In metabolic alkalosis (as with nasogastric suction or vomiting), bicarbonaturia obligates excretion of a cation. Therefore, the urine sodium concentration may not be low in cases of volume contraction. In this circumstance, the urine chloride concentration should be measured. The urine chloride concentration will typically be less than 15 mEq/L in cases of volume contraction.
• In states of low effective arterial volume (heart failure and cirrhosis), there may be avid kidney tubular sodium reabsorption even in cases of severe hypervolemia. Physical findings of hypervolemia will usually suggest that such patients need to be diuresed.
• If a patient with acute kidney injury is being treated with diuretics, the urine sodium concentration may not be low, even in the presence of volume contraction. In this circumstance, the fractional excretion of urea should be calculated. A fractional excretion of urea of less than 35% suggests the presence of volume contraction and prerenal azotemia.
• If volume contraction and acute tubular necrosis coexist. Because kidney tubular function is impaired, the urine sodium concentration may not be low.
