Measuring Rat Serum Osmolality by Freezing Point Osmometry

Blood serum or plasma osmolality is the measure of electrolyte to water balance in the body’s circulation, and is tightly regulated in physiological states in order to maintain normal levels of serum solute (Bourque, 2008). Osmolality is defined as the number of osmoles of solute per kg of water (mOsm/kg) (Dufour, 1993) and can be measured using different techniques that rely on the colligative properties of the solution. The most commonly used in lab settings are vapour pressure and freezing point osmometry, which are relatively quick and easy to perform. Freezing point osmometry is preferred because it is insensitive to volatile compounds, such as alcohol, that may be present in the solution. Measurement of serum or plasma osmolality is clinically relevant for a number of conditions and diseases, including hypernatremia, diabetic ketoacidosis, and the syndrome of inappropriate antidiuresis (Ellison, 2013; Lupsa and Inzucchi, 2013; Reddi, 2013). In this protocol, we describe the measurement of serum osmolality in rats using the freezing point osmometry technique as originally outlined in our previous study of osmoregulatory perturbations in sepsis (Stare et al., 2015).