2Na + Glucose/18 + BUN/2.8
The Serum Osmolality Calculator estimates a patient's calculated serum osmolality from three routine labs — sodium, glucose, and blood urea nitrogen (BUN) — using the standard calculated-osmolality formula. It also supports an optional osmolar gap calculation when a lab-measured osmolality is available, which helps flag unmeasured osmotically active substances such as toxic alcohols. The tool is built around a normal reference range of 270–295 mOsm/kg and provides bedside interpretation for hypo-osmolar and hyperosmolar results.
It uses the standard calculated-osmolality equation: 2 × Na + Glucose/18 + BUN/2.8, with sodium in mEq/L and glucose/BUN in mg/dL.
The app defines normal as 270–295 mOsm/kg; values below this range are flagged hypo-osmolar and values above are flagged hyperosmolar.
It is the difference between a lab-measured osmolality and the calculated value; the tool flags a gap over 10 mOsm/kg as suspicious for an unmeasured osmole such as a toxic alcohol.
The app's reference notes specify using total (uncorrected) sodium in the formula, not a sodium value corrected for glucose.
The app lists dehydration, hypernatraemia, hyperglycaemia (DKA/HHS), and mannitol therapy as causes, and advises correcting slowly to avoid cerebral oedema.