What it is
Calculated serum osmolality estimates plasma osmolality from its major solutes. Subtracting it from a measured (freezing-point depression) osmolality gives the osmolar gap — a screen for unmeasured osmotically active substances, classically toxic alcohols.
Formula
This tool uses US conventional units (glucose and BUN in mg/dL):
$$ Osm_{calc} = 2 \times Na + \frac{glucose}{18} + \frac{BUN}{2.8} + \frac{ethanol}{3.7} $$
The 2×Na term accounts for sodium and its accompanying anions; the divisors convert mg/dL to mmol/L for glucose (18), urea nitrogen (2.8), and ethanol (3.7). Ethanol is optional — include it when measured so a known alcohol does not masquerade as an unexplained gap. For SI units use 2×Na + glucose + urea (all mmol/L) instead.
When to use
- Suspected toxic alcohol ingestion (methanol, ethylene glycol) — especially early, before an anion-gap acidosis develops.
- Unexplained altered mental status, high anion gap, or a discrepancy between clinical picture and routine chemistry.
Worked example
Na 140, glucose 90 mg/dL, BUN 14 mg/dL, no ethanol entered.
$$ Osm_{calc} = 2(140) + \frac{90}{18} + \frac{14}{2.8} = 280 + 5 + 5 = 290\ \text{mOsm/kg} $$
That falls in the typical 275–295 mOsm/kg range. If the lab-measured osmolality were 315, the osmolar gap = 315 − 290 = 25 — well above 10, flagging an unmeasured osmole and prompting a toxic-alcohol workup.
Pitfalls
- A normal gap does not exclude toxic alcohol. As methanol/ethylene glycol are metabolised to acids, the gap falls while the anion gap rises — late presenters can have a normal osmolar gap and a deadly acidosis. Interpret gap and anion gap together over time.
- The “normal” gap range is wide (often cited −10 to +10); a gap of 10 in someone with a low baseline can still be significant. There is no single safe cutoff.
- Account for ethanol. An unaccounted ethanol level inflates the gap and mimics another osmole — enter it when known.
- Use the matching units. This formula is conventional-unit (mg/dL); mixing in SI values corrupts the result.
- The gap is a screen — definitive diagnosis needs specific drug/alcohol assays.
Run it: Calculated Serum Osmolality
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.