Osmolality - serum

Last updated: Monday, 22, March, 2004

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5 mL blood in plain tube.


Freezing point depression; vapour pressure osmometers are unsuitable for detecting solvents.

Calculated osmolality (mmol/L) = 1.86 (Na + K) + glucose + urea + 10.

Osmolar gap = measured osmolality minus calculated osmolality.

Reference Interval

Neonate: 270-290 mmol/kg

Adult: 280-300 mmol/kg


Suspected poisoning with alcohol, methanol, ethylene glycol, acetone, isopropanol, diethyl ether or paraldehyde.

Rarely of assistance for the assessment of water and electrolyte balance in conjunction with urine osmolality.

This test is not routinely indicated in hypernatraemia, hyper­glycaemia, uraemia or hyponatraemia.


An increased osmolar gap indicates the presence of alcohol or other osmotically active substances.

Urine osmolality measurements may assist interpretation. See also Osmolality - urine.


Penney MD and Walters G. Ann Clin Biochem 1987; 24: 566-571.