What urine sodium (Na) labs help determine the type of hyponatremia a patient has?

From the Guidelines

To determine the type of hyponatremia a patient has, measure urine sodium concentration, as a random “spot” urine sodium concentration that is greater than the potassium concentration correlates with a 24-hour sodium excretion greater than 78 mmol/day with approximately 90% accuracy 1.

Key Laboratory Tests

• Urine sodium concentration: helps distinguish between hypovolemic, euvolemic, and hypervolemic hyponatremia
• Urine osmolality: helps distinguish between appropriate and inappropriate ADH secretion
• Fractional excretion of sodium (FENa): provides additional information on volume status
• Serum osmolality: confirms true hyponatremia versus pseudohyponatremia

Clinical Assessment

• Examine for edema, orthostatic hypotension, skin turgor to assess volume status
• Consider the patient's medical history, including liver disease, heart failure, and syndrome of inappropriate antidiuretic hormone secretion (SIADH)

Treatment Strategies

• Hypovolemic hyponatremia: administer sodium and identify the causative factor
• Hypervolemic hyponatremia: induce a negative water balance, consider fluid restriction, and use vaptans (e.g., tolvaptan, conivaptan) to improve serum sodium concentration 1
• Euvolemic hyponatremia: consider treatment with vaptans or other medications, depending on the underlying cause

Recent Guidelines

• The 2018 guidelines from the Clinical and Molecular Hepatology recommend restricting fluid intake to 1.0-1.5 L/day in cases of dilutional hyponatremia when the serum sodium concentration falls below 120-125 mmol/L 1
• The 2010 guidelines from the Journal of Hepatology recommend using vaptans to improve serum sodium concentration in patients with hypervolemic hyponatremia, with close monitoring to avoid complications such as hypernatremia and dehydration 1

From the Research

Urine Na Labs to Help Decide Type of Hyponatremia

To determine the type of hyponatremia a patient has, several urine labs can be helpful, including:

• Urine sodium concentration: This can help differentiate among the possible causes of hyponatremia, as a low urine sodium level (<20 mmol/L) is indicative of hypovolemia, whereas a high level (>40 mmol/L) is suggestive of the syndrome of inappropriate antidiuretic hormone secretion 2
• Urine osmolality: This can be used to determine whether water excretion is normal or impaired, with a value of <100 mOsm/kg indicating complete and appropriate suppression of antidiuretic hormone secretion 2
• Fractional uric acid excretion: This is a recently identified parameter that may further improve the diagnostic approach to hyponatremia 3
• Plasma copeptin concentration: This is another recently identified parameter that may help in the diagnosis of hyponatremia 3

Laboratory Evaluation

The laboratory evaluation of hyponatremia typically involves:

• Measurement of the effective serum tonicity (serum osmolality less serum urea level) to determine if the patient has a hypo-osmolar state (<280 mOsm/kg) 2
• Measurement of serum and urine osmolality to determine if water excretion is normal or impaired 2, 4
• Measurement of urine electrolytes, including sodium, to help determine the underlying cause of hyponatremia 2, 4

Clinical Assessment

In addition to laboratory evaluation, clinical assessment is also important in determining the type of hyponatremia a patient has, including:

• Estimation of volume status using physical examination and point-of-care ultrasonography 4
• Assessment of concurrent illness and medication use, as these can be contributing factors to hyponatremia 2, 4