Serum Osmolality Calculator

The Serum Osmolality Calculator computes expected serum osmolality from sodium, glucose, and BUN, and determines the osmolal gap when a measured osmolality value is provided. Osmolality represents the concentration of osmotically active solutes per kilogram of solvent and is a critical parameter in evaluating fluid balance, hyponatremia, hypernatremia, and suspected toxic ingestions.

The calculated serum osmolality uses the formula: Calculated Osmolality = 2 x Na + Glucose/18 + BUN/2.8. Sodium is doubled to account for its accompanying anions (primarily chloride and bicarbonate). Glucose and BUN are divided by their respective molecular weight divided by 10 to convert from mg/dL to mOsm/kg. Normal calculated osmolality is approximately 275-295 mOsm/kg. BUN contributes to measured osmolality but is considered an ineffective osmole because urea crosses cell membranes freely and does not create osmotic gradients.

The osmolal gap is the difference between measured osmolality (determined by freezing point depression in the laboratory) and calculated osmolality. A normal osmolal gap is less than 10 mOsm/kg. An elevated osmolal gap indicates the presence of osmotically active substances not accounted for in the calculation formula, most importantly toxic alcohols such as methanol and ethylene glycol, as well as ethanol, isopropyl alcohol, mannitol, and propylene glycol.

The osmolal gap is a crucial tool in the emergency evaluation of suspected toxic alcohol ingestion. Methanol and ethylene glycol are initially present as uncharged osmotically active molecules that raise the measured osmolality without affecting the calculated osmolality, producing an elevated osmolal gap. As these parent alcohols are metabolized by alcohol dehydrogenase to their toxic acid metabolites (formic acid and glycolic/oxalic acid, respectively), the osmolal gap decreases while the anion gap increases. This temporal relationship between osmolal gap and anion gap is pathognomonic for toxic alcohol poisoning.

Ethanol is the most common cause of an elevated osmolal gap. Each 100 mg/dL (21.7 mmol/L) of ethanol contributes approximately 22 mOsm/kg to measured osmolality. When evaluating the osmolal gap for toxic alcohol ingestion, the ethanol contribution should be subtracted: Adjusted Osmolal Gap = Measured Osm - Calculated Osm - (Ethanol level / 4.6). If the adjusted gap remains elevated, toxic alcohol ingestion should be strongly considered.

Important limitations of the osmolal gap include its moderate sensitivity; a normal osmolal gap does not exclude toxic alcohol ingestion, particularly if significant time has elapsed and the parent alcohol has been metabolized to acid metabolites. Additionally, the baseline osmolal gap varies among individuals (range approximately -10 to +10), so a patient with a baseline gap of -5 who ingests methanol may have a measured gap of only 8 despite clinically significant poisoning. Clinical suspicion should drive evaluation regardless of a borderline gap.