Evaluation of Hyponatremia Using Serum Osmolality and Extracellular Fluid Volume

The cause of hyponatremia can be narrowed to a few possibilities by looking at clinical, historical and laboratory findings.

serum osmolality	secondary test	tertiary test	Possible Cause for Hyponatremia
elevated	elevated serum glucose		hyperglycemia
	history of hypertonic infusion		infusion of glucose, mannitol or contrast agents
normal	elevated serum protein		pseudohyponatremia due to hyperproteinemia
	elevates serum lipids		pseudohyponatremia due to hyperlipidemia
	history of isotonic infusion		infusion of glucose or mannitol
decreased	ECF volume elevated	urine sodium > 20 mEq/L	acute renal failure chronic renal failure
	ECF volume elevated	urine sodium < 10 mEq/L	cirrhosis congestive heart failure nephrosis
	ECF volume normal		water intoxication SIADH reset osmostat stress (pain, emotion, post-op) drugs hypothyroidism potassium loss glucocorticoid deficiency
	ECF volume decreased	urine sodium > 20 mEq/L	diuretics, osmotic diuretics (urea, mannitol, glucose) urinary obstruction salt-losing nephritis ketonuria bicarbonaturia (RTA, metabolic alkalosis) adrenal insufficiency
	ECF volume decreased	urine sodium < 10 mEq/L	GI losses (diarrhea, vomiting, NG suction) skin loss (burns, sweating) lung losses third space loss traumatized muscle post-diuretic therapy

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