Osmometry

Osmometry measures the osmotic strength of the particles within a sample. In clinical labs, this is most commonly performed using freezing point depression methodology.

Methodologies

Various techniques can be used to measure osmolality.

Technique	Mechanism	Strengths	Limitations
Freezing Point	Measures solute by freezing the sample and determining the freezing point.	Accurate Can be used to measure volatile solutes
Vapour Pressure	Measures the decrease in vapour pressure when more solute is present.		Unreliable when measuring volatile compounds (e.g., alcohols, acetone, etc.)

Uses

There are a variety of uses for osmometry and made include serum osmolality, urine osmolality, or both.

• Investigating suspected toxin ingestion:
  • Alcohols (ethanol, methanol, isopropanol)
  • Ethylene and propylene glycol (antifreeze)
  • ASA and other salicylic acids
  • Paraldehyde
• Investigation of renal function
• Investigation of diabetes mellitus or diabetes insipidus
• Monitoring osmotically active compounds:
  • Mannitol (as treatment for cerebral edema or when used for surgical irrigation)
• Assessment of hyponatremia
  • Can differentiate true hyponatremia from pseudohyponatremia

Serum Osmolality	RI: 275 - 295 mOsm/kg
	▲ Increased	▼ Decreased
Conditions	Toxin ingestion: Alcohols (ethanol, methanol, isopropanol) Ethylene and propylene glycol (antifreeze) ASA and other salicylic acids Paraldehyde Monitoring osmotically active compounds: Mannitol (as treatment for cerebral edema) Therapeutic: osmolar gap ~10 mOsm/kg Renal toxicity: osmolar gap ≥50 mOsm/kg
Lab Interferences
Other Factors