The Cardiovascular
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Serum Osmolality / Osmolarity

Calculated serum osmolality from sodium, glucose, and urea.

Serum Osmolality / Osmolarity
Sodium
mEq/L
Glucose
Blood urea nitrogen (BUN)
mg/dL
Complete the fields above to see the result.

When to use

  • Computing the osmolar gap when toxic alcohol (methanol, ethylene glycol) ingestion is suspected.
  • Assessing effective osmolality in hyper- or hyponatraemic states.

Formula

Calculated osmolality = 2 x Na + glucose/18 + BUN/2.8, with glucose and BUN in mg/dL. In SI units it is 2 x Na + glucose + urea (all mmol/L).

Pearls and pitfalls

  • Add ethanol/3.7 (mg/dL) when significant ethanol is present to avoid overcalling the osmolal gap.
  • The osmolal gap requires a laboratory-measured osmolality; this tool provides only the calculated value.

References

  1. Dorwart WV, Chalmers L. Clin Chem. 1975;21(2):190-4.