What is Serum Osmolality?
Serum osmolality quantifies the number of osmotically active particles per kilogram of solvent in blood. It depends primarily on sodium concentration, which accounts for roughly 80% of serum osmolality, with smaller contributions from urea and glucose. Minor ions like potassium have negligible impact.
Osmolality differs subtly from osmolarity: osmolality uses solvent weight as the denominator (measured in mOsm/kg H₂O), while osmolarity uses total solution volume (measured in mOsm/L). In clinical practice, the values are nearly identical at physiologic concentrations, but osmolality remains the standard for laboratory measurement via freezing-point depression.
Normal serum osmolality ranges from 275–295 mOsm/kg H₂O. Abnormal values signal either excessive water relative to solutes (low osmolality) or insufficient water relative to solutes (high osmolality).
Serum Osmolality Calculation
The modified Pursell formula below incorporates ethanol and methanol, improving accuracy for poisoning cases:
Serum osmolality = (2 × Na) + (BUN ÷ 2.8) + (glucose ÷ 18) + (alcohol ÷ 3.7)
Na— Serum sodium concentration (mEq/L or mmol/L)BUN— Blood urea nitrogen (mg/dL)glucose— Blood glucose concentration (mg/dL)alcohol— Serum ethanol, methanol, or ethylene glycol concentration (mg/dL)
Osmotic Gap and Clinical Interpretation
The osmotic gap is the difference between measured serum osmolality (obtained directly from the laboratory using an osmometer) and calculated osmolality:
Osmotic gap = measured osmolality − calculated osmolality
A normal osmotic gap ranges from −14 to +10 mOsm/kg H₂O. Values exceeding this range suggest the presence of unmeasured osmotically active substances such as:
- Toxins: methanol, ethylene glycol, propylene glycol
- Medications: mannitol, contrast agents
- Endogenous solutes: in severe hyperproteinaemia or hypertriglyceridaemia
An elevated osmotic gap in the context of anion gap metabolic acidosis classically raises suspicion for toxic alcohol ingestion and warrants urgent toxicology screening.
High and Low Serum Osmolality: Causes
High serum osmolality (>295 mOsm/kg) indicates relative water depletion or solute excess:
- Dehydration from inadequate fluid intake or excessive losses (vomiting, diarrhoea, sweating)
- Diabetes insipidus (central or nephrogenic) with inability to concentrate urine
- Hyperglycaemia in uncontrolled diabetes
- Hypernatraemia from water loss or excessive sodium intake
- Acute kidney injury reducing fluid excretion
Low serum osmolality (<275 mOsm/kg) indicates relative water excess or solute depletion:
- Excessive free water intake or hypotonic fluid administration
- SIADH (syndrome of inappropriate antidiuretic hormone secretion) from malignancy, CNS disease, or medications
- Hyponatraemia from sodium loss or dilution
- Liver cirrhosis with impaired solute metabolism
- Paraneoplastic syndromes causing autonomous ADH secretion
Clinical Considerations
When interpreting serum osmolality, remember these practical caveats:
- Timing of alcohol measurement matters — Serum ethanol and methanol concentrations change rapidly with metabolism (approximately 10–20 mg/dL per hour). A delayed osmotic gap calculation may underestimate recent ingestion. Always note the exact time of blood draw and suspected ingestion timing.
- Unit conversions affect accuracy — BUN and glucose must be in mg/dL for the formula to work correctly. If laboratory values are reported in mmol/L, convert BUN by multiplying by 2.8 and glucose by multiplying by 18 before substituting into the equation.
- Pseudohyponatraemia and protein effects — Severe hyperproteinaemia (>10 g/dL) or hypertriglyceridaemia (>1500 mg/dL) can artificially lower measured sodium on certain analyzers. The osmotic gap may appear falsely elevated in these cases—direct measurement of osmolality helps clarify whether unmeasured solutes or analytic error is responsible.
- Sample stability and handling — Serum osmolality samples can drift if left at room temperature; specimens should be refrigerated and sent promptly to the laboratory. Haemolysed, icteric, or lipemic samples may affect the freezing-point osmometer reading.