Chemistry
Calculated Serum Osmolality
Estimated osmolality from electrolytes.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
Calculated serum osmolality estimates the concentration of osmotically active particles in plasma from routine electrolytes, using the formula 2 × sodium + glucose + urea (all in mmol/L). Sodium (with its accompanying anions) is the dominant contributor, with glucose and urea adding smaller amounts. The calculated value is compared with a directly measured osmolality to derive the osmolar gap, which reflects unmeasured osmoles. It is most useful in toxicology and in the assessment of unexplained metabolic disturbance.
Interpreting the result
A normal calculated serum osmolality is approximately 275–295 mOsm/kg; values below this are low and values above are high. The key clinical use is the osmolar gap — the difference between measured and calculated osmolality — which is normally less than about 10. A raised osmolar gap indicates unmeasured osmoles such as methanol, ethylene glycol, ethanol or mannitol, and in the right context should prompt urgent toxicology assessment, whereas a normal gap makes a significant toxic-alcohol ingestion less likely.
Worked example
A patient has sodium 140 mmol/L, glucose 6 mmol/L and urea 5 mmol/L: calculated osmolality = (2 × 140) + 6 + 5 = 291 mOsm/kg. If the laboratory-measured osmolality is 320 mOsm/kg, the osmolar gap is about 29 — clearly raised, prompting a search for unmeasured osmoles such as a toxic alcohol.
Critical actions
Calculated osmolality = 2×Na + glucose + urea (mmol/L). Compare with a measured osmolality to calculate the osmolar gap (normal <10).
Pearls / pitfalls
- A normal osmolar gap does not exclude a toxic-alcohol ingestion, especially late after exposure once the parent alcohol has been metabolised to acids.
- Ethanol is a common cause of a raised osmolar gap and should be accounted for before attributing the gap to a more dangerous toxin.
- Use consistent units — the mmol/L formula here differs from US formulae that express glucose and urea (BUN) in mg/dL.
- The osmolar gap is a screening clue, not a diagnosis; correlate with anion gap, acid-base status and the clinical history.
Evidence & validation
The 2 × Na + glucose + urea formula is a long-standing, widely used approximation of measured osmolality. The osmolar gap is a recognised screening adjunct in toxicology guidance, used together with the clinical picture and acid-base status rather than in isolation.
Frequently asked questions
What is the osmolar gap?
It is the difference between the directly measured serum osmolality and the calculated value. A gap greater than about 10 suggests unmeasured osmoles are present, such as a toxic alcohol, mannitol or ethanol.
What does a high osmolar gap suggest?
It points to unmeasured osmotically active substances in the blood, classically methanol or ethylene glycol, but also ethanol, mannitol or isopropanol. In the right clinical setting it should prompt urgent toxicology assessment.
Can a normal gap rule out toxic-alcohol poisoning?
No. Late after ingestion the parent alcohol may already have been metabolised to organic acids, normalising the osmolar gap while causing a high anion-gap metabolic acidosis. Both the gap and the acid-base picture must be considered.
Why do different formulae give different numbers?
Formulae vary by units and by whether they include ethanol or a correction factor. The version here uses mmol/L for sodium, glucose and urea; US versions using mg/dL for glucose and BUN look different but estimate the same quantity.
References
- Bhagat CI, Garcia-Webb P, Fletcher E, Beilby JP. Calculated vs measured plasma osmolalities revisited. Clin Chem. 1984;30(10):1703–1705.
- Krasowski MD, et al. A retrospective analysis of the use of the osmolal gap. BMC Clin Pathol. 2012;12:1.
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