Osmometry (freezing point depression)

Principle

• Osmolality is the number of dissolved solute particles per kilogram of water (mOsm/kg). It is a colligative property, depending on particle number, not type.
• Colligative properties: freezing point depression, vapour pressure lowering, boiling point elevation and osmotic pressure increase.
• One osmole of solute per kilogram of water lowers the freezing point by 1.86 °C.
• In a freezing point osmometer the sample is supercooled, then freezing is induced by vibration or stirring; released heat of fusion raises temperature to a plateau (the freezing point), measured by a thermistor.

Specimen

• Serum (or heparin plasma per manufacturer) and random or timed urine. Avoid evaporation; keep tubes capped.

Results

• Typical serum osmolality 275–295 mOsm/kg; urine about 50–1200 mOsm/kg depending on hydration. Local ranges apply.
• Calculated osmolality ≈ 2 × Na + glucose/18 + BUN/2.8 (mg/dL units), or 2 × Na + glucose + urea (mmol/L).
• Osmolal gap = measured – calculated; typically below 10 mOsm/kg.

Interpretation

• Raised osmolal gap: ethanol, methanol, ethylene glycol, isopropanol, mannitol, acetone.
• Low serum osmolality with inappropriately concentrated urine suggests SIADH.
• High serum osmolality with dilute urine suggests diabetes insipidus.
• Pseudohyponatraemia (lipids or proteins) shows normal measured osmolality.

Quality control and pitfalls

• Vapour pressure osmometers do not detect volatile solutes such as alcohols; freezing point osmometers do, so use freezing point for osmolal gap.
• Calibrate with NaCl standards; run controls; check sample volume, air bubbles and particles, which cause premature freezing.

Clinical use

• Evaluation of hyponatraemia, hypernatraemia, polyuria, water deprivation testing and toxic alcohol poisoning.