There are three essential laboratory tests in the evaluation of patients with hyponatremia that, together with the history and the physical examination, help to establish the primary underlying etiologic mechanism: urine osmolality, serum osmolality, and urinary sodium concentration.
Urine osmolality
Urine osmolality helps differentiate between conditions associated with impaired free-water excretion and primary polydipsia. A urine osmolality greater than 100 mOsm/kg indicates impaired ability of the kidneys to dilute the urine.
Serum osmolality
Serum osmolality readily differentiates between true hyponatremia and pseudohyponatremia. The latter may be secondary to hyperlipidemia or hyperproteinemia, or may be hypertonic hyponatremia associated with elevated glucose, mannitol, glycine (posturologic or postgynecologic procedure), sucrose, or maltose (contained in IgG formulations).
Urinary sodium concentration
Urinary sodium concentration helps differentiate between hyponatremia secondary to hypovolemia and syndrome of inappropriate antidiuretic hormone secretion (SIADH). With SIADH (and salt-wasting syndrome), the urine sodium is greater than 20-40 mEq/L. With hypovolemia, the urine sodium typically measures less than 25 mEq/L. However, if sodium intake in a patient with SIADH (or salt-wasting) happens to be low, then urine sodium may fall below 25 mEq/L.