Measured serum osmolality and sodium in causes of hyponatremia

Condition	Measured serum osmolality	Serum sodium by autoanalyzer	Serum sodium by direct ion-selective electrode
Normal patient	Normal	Normal	Normal
Hypotonic hyponatremia	Low	Low	Low
Hyperglycemia or intravenous mannitol or immune globulin	High	Low	Low
Absorbed surgical irrigants	Normal*	Low	Low
Pseudohyponatremia due to hyperlipidemia or hyperproteinemia	Normal	Low	Normal

Bedside blood gas analyzers measure serum sodium using a direct ion-selective electrode. Occasionally, laboratories will have an autoanalyzer that uses a direct ion-selective electrode.

* After absorption of large volumes of 1.5% glycine, the serum osmolality may be only slightly reduced, with an extremely low serum sodium concentration.

Courtesy of Richard H Sterns, MD.

Graphic 116018 Version 2.0

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Measured serum osmolality and sodium in causes of hyponatremia