Laboratory & Electrolytes

Serum Osmolality

Calculate estimated serum osmolality and osmol gap to detect unmeasured osmoles.

When to use: Calculate serum osmolality to evaluate hyponatraemia (step in workup — distinguishes hypo-osmolar from iso/hyperosmolar causes), to assess degree of hyperosmolality in DKA and HHS, and to estimate the osmol gap when toxic alcohol ingestion is suspected. An osmol gap >10 mOsm/kg indicates unmeasured osmoles (ethanol, methanol, ethylene glycol, isopropanol, mannitol) and is a key emergency medicine and toxicology tool.

Calculator

Serum Sodium (Na⁺)(mEq/L)

Serum Glucose(mg/dL)

Blood Urea Nitrogen (BUN)(mg/dL)BUN (not urea). If urea in mmol/L: BUN mg/dL = urea mmol/L × 2.8

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Formula

Serum Osmolality = 2 × Na (mEq/L) + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8 Normal: 285–295 mOsm/kg | Low <285 (amber) | >295 (amber) | >320 = Critical (red) Osmol gap = Measured − Calculated: >10 suggests toxic alcohols (methanol, ethylene glycol, ethanol)

Key Points for NEET PG

Formula: 2Na + glucose/18 + BUN/2.8 — the most important osmolality formula in clinical medicine.
Osmol gap = Measured osmolality (by freezing-point depression) − Calculated: >10 mOsm/kg suggests ethanol, methanol, ethylene glycol, mannitol.
HHS: serum osmolality >320 mOsm/kg with glucose >600 mg/dL and minimal ketosis — differentiates from DKA.
Hyponatraemia classification: hypoosmolar (<280) vs isoosmolar (280–295, pseudohyponatraemia) vs hyperosmolar (>295, hyperglycaemia, mannitol).
BUN is an "ineffective osmole" — it crosses cell membranes freely and does not cause water shifts; hence effective osmolality = 2Na + glucose/18.

References

Bhave G, Neilson EG. Body fluid dynamics: back to the future · J Am Soc Nephrol (2011)

Kruse JA, Cadnapaphornchai P. The serum osmole gap · J Crit Care (1994)

For educational purposes only. Not for clinical decision-making without professional oversight.