Hyponatremia Workup

Updated: Jun 17, 2019
  • Author: Eric E Simon, MD; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Workup

Laboratory Studies

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. (In general, the etiology of the hyponatremia directs its management.) These tests are as follows

  • Urine osmolality

  • Serum osmolality

  • Urinary sodium concentration

Urine osmolality helps to differentiate between conditions associated with impaired free water excretion and primary polydipsia, in which water excretion should be normal (provided intact kidney function). With primary polydipsia, as with malnutrition (severe decreased solids intake) and a reset osmostat, the urine osmolality is maximally dilute, generally less than 100 mOsm/kg. A urine osmolality greater than 100 mOsm/kg indicates impaired ability of the kidneys to dilute the urine. This usually is secondary to elevated vasopressin (antidiuretic hormone; ADH) levels, appropriate or inappropriate.

Serum osmolality readily differentiates between true hyponatremia and pseudohyponatremia secondary to hyperlipidemia, hyperproteinemia, or hypertonic hyponatremia. Sources of hypertonic hyponatremia include elevations of the following:

  • Glucose

  • Mannitol

  • Glycine (after urologic or gynecologic procedures)

  • Sucrose

  • Maltose (contained in IgG formulations)

Urinary sodium concentration helps to differentiate between hyponatremia secondary to hypovolemia and the syndrome of inappropriate ADH 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.

Ancillary tests

Serum uric acid levels can be important supportive information (they are typically reduced in SIADH and also reduced in salt wasting). After correction of hyponatremia, the hypouricemia corrects in SIADH but remains with a salt-wasting process.

Thyroid-stimulating hormone (TSH) and serum cortisol levels should be measured if hypothyroidism or hypoadrenalism is suspected.

Serum albumin, triglycerides, and a serum protein electrophoresis also may be indicated for particular patients.

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Imaging Studies

Head computed tomography (CT) scanning and chest radiography can be used to assess for an underlying etiology in select patients with suspected SIADH or cerebral salt wasting.

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