Which property is used to measure osmolality in osmometers for urine or serum?

• A. freezing point
• B. sedimentation point
• C. midpoint
• D. osmotic pressure

Answer: (A)

Osmolality is determined using a colligative property: how solute particles depress the freezing point of a solvent. In a freezing point osmomter, the sample (urine or serum) is cooled until ice begins to form, and the instrument records the temperature at which freezing starts. The more dissolved particles present, the lower that freezing point. Because the freezing-point depression is directly related to the number of osmotically active particles, the measured depression is converted into an osmolality value (in mOsm/kg). This is the basis for why freezing point is the property used in these devices. Sedimentation point or midpoint aren’t relevant to osmolality measurement, and osmotic pressure is not the standard direct readout in routine clinical osmometers (though related concepts exist in other osmometry approaches).