Weill Cornell Medicine Care Discover Teach
Weill Cornell Medicine — Pediatric Critical Care Medicine

Serum Osmolality — Measured

All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.

Calculate predicted serum osmolality from sodium, glucose, and BUN. Optionally enter the measured osmolality to compute the osmolal gap.

mEq/L
mg/dL
mg/dL

For osmolal gap, also enter the measured osmolality:

mOsm/kg
mOsm/kg

The predicted serum osmolality is calculated from the three major osmotically active solutes in plasma:

Osmolality  =  2 × Na  +  Glucose 18  +  BUN 2.8

Sodium is multiplied by 2 to account for its accompanying anions. Glucose (mg/dL) is divided by 18 to convert to mmol/L (molecular weight 180 ÷ 10). BUN (mg/dL) is divided by 2.8 to convert to mmol/L (urea molecular weight 28 ÷ 10). Normal range is 285–295 mOsm/kg.

The osmolal gap is the difference between the laboratory-measured osmolality (by freezing point depression) and the calculated value. A gap greater than 10 mOsm/kg is considered elevated and suggests the presence of additional osmotically active substances not accounted for by the formula.

Common causes of an elevated osmolal gap include:

  • Ethanol (most common)
  • Methanol
  • Ethylene glycol
  • Isopropanol
  • Mannitol
  • Propylene glycol — particularly from prolonged lorazepam infusions, where it may cause hyperosmolarity and, rarely, hyperosmolar coma
  • Severe hyperproteinaemia or hyperlipidaemia (pseudohyponatraemia)

Note that urea (BUN) crosses cell membranes freely and does not contribute to effective (tonicity) osmolality. Some formulations of the osmolality equation therefore omit the BUN term when assessing tonicity. The formula above calculates total osmolality (including BUN), consistent with the measured value by freezing point depression.

References

  1. Bhagat CI, Garcia-Webb P, Fletcher E, Beilby JP. Calculated vs measured plasma osmolalities revisited. Clin Chem. 1984;30(10):1703–1705.
  2. Kruse JA, Cadnapaphornchai P. The serum osmol gap. J Crit Care. 1994;9(3):185–197.
  3. Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008;8:5.