Your calculated serum osmolality reflects the estimated total concentration of dissolved particles in the blood based on sodium, glucose, and BUN — the three major osmotically active solutes. A normal calculated osmolality is 275–295 mOsm/kg. Values below 275 suggest hypo-osmolality, which is most commonly associated with hyponatremia and can cause cerebral edema and neurological symptoms. Values above 295 suggest hyperosmolality, seen in dehydration, diabetic ketoacidosis (DKA), hyperglycemic hyperosmolar state (HHS), and toxic ingestions.

The calculated osmolality becomes especially powerful when compared to the measured osmolality from the laboratory. The difference between measured and calculated osmolality is called the osmolal gap. A gap greater than 10 mOsm/kg suggests the presence of unmeasured osmotically active substances — most importantly toxic alcohols (methanol, ethylene glycol, isopropyl alcohol) but also ethanol, mannitol, or contrast dye. This makes the osmolal gap a critical tool in the emergency toxicology workup.

Use the calculated serum osmolality whenever you need to assess a patient's overall osmolar status or when you suspect the presence of unmeasured osmoles. The most important clinical applications include the workup of suspected toxic alcohol ingestion (methanol, ethylene glycol, isopropyl alcohol), evaluation of hyponatremia (to distinguish true hypotonic hyponatremia from pseudohyponatremia and hypertonic hyponatremia), and assessment of hyperosmolar states such as DKA and HHS.

In the emergency department, calculating the osmolal gap is a standard step when a patient presents with altered mental status, unexplained metabolic acidosis with an elevated anion gap, or a history concerning for toxic ingestion. It is also used in the ICU for monitoring patients receiving mannitol or hypertonic saline therapy.

The calculated osmolality is an estimate that accounts only for sodium, glucose, and BUN. It does not capture the contribution of other osmotically active substances that may be present in the blood, which is precisely why the osmolal gap exists as a diagnostic tool. However, the accuracy of the osmolal gap depends on the precision of both the measured and calculated values, and small errors in either can produce misleading results.

Different formulas for calculating osmolality exist, and they can yield slightly different results. This tool uses the most commonly cited formula (2×Na + Glucose/18 + BUN/2.8), but some institutions use variations that include ethanol as a fourth term. Additionally, a normal osmolal gap does not completely rule out toxic alcohol ingestion — in late presentations, the parent alcohol may have been metabolized to its acid metabolites (e.g., formic acid from methanol), which do not contribute to the osmolal gap but do raise the anion gap. The osmolal gap and anion gap should be interpreted together in suspected poisoning cases.