Osmolality

Keywords: Osmotic gap, Osm Gap, Osmolar gap

Description:

Osmolality is the concentration per weight solvent (mmol/kg)

Osmolarity is the concentration solute per volume solvent (mmol/L)

Osmolal gap is the measured Osmolality minus Osmolarity (mmol/kg)

Specimen:

5 mL blood in plain tube.

Method:

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

A number of formulas have been used to calculate osmolality, some of which include potassium in the calculation. For example:

Calculated Osmolarity (mmol/L) = 1.86 (Na + K) + Glucose + Urea + 10.

Osmotic gap = measured Osmolality minus calculated Osmolarity.

Reference Interval:

Neonate: 270-290 mmol/kg

Adult: 275-295 mmol/kg

Application:

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, Hyponatraemia, hyper­glycaemia or uraemia.

Interpretation:

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

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

Reference:

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

Bhagat CI et al. Clin Chem 1984; 30: 1703-1705.

Rose BD and Post TW. Clinical physiology of acid base and electrolyte disorders. 2001.