Plasma Osmolality, Osmolal Gap, and Osmometry: Principles, Measurement, and Clinical Interpretation

Osmometry is the determination of the concentration of solute particles in a solution based on their contribution to osmotic pressure. In biological systems, osmotic pressure governs the movement of water across semipermeable membranes that separate two solutions of differing solute concentrations. This process is fundamental to the maintenance of cellular volume, extracellular fluid balance, and overall homeostasis.

The permeability of biological membranes varies according to pore size, molecular shape, and charge. Selective membranes such as the glomerular filtration barrier of the kidney and capillary endothelium are freely permeable to water and most small ions and molecules, including sodium, chloride, glucose, and urea. In contrast, larger molecules such as plasma proteins are largely excluded.

When osmotically active solutes that cannot cross a membrane differ in concentration between compartments, an osmotic gradient is established. Water and other permeable molecules move across the membrane to reduce this gradient until osmotic equilibrium is achieved. The resulting osmotic pressure reflects the total number of solute particles, rather than their molecular identity or chemical properties.

Colligative properties

When a solute is added to a solvent, it alters several physical properties of the solution. These changes occur because solute particles reduce the vapor pressure of the solvent, leading to predictable effects that depend solely on the number of dissolved particles. These effects are known as colligative properties and include:

1. Increased osmotic pressure,
2. Lowered vapor pressure,
3. Increased boiling point, and
4. Decreased freezing point

Electrolytes exert a greater colligative effect than nonelectrolytes because they dissociate into multiple ions in solution. For example, sodium chloride dissociates into two particles (Na⁺ and Cl⁻), while calcium chloride dissociates into three. However, real solutions often deviate from ideal behavior due to incomplete dissociation and interactions between solute and solvent molecules.

To account for these deviations, the osmotic coefficient (ϕ) is used. This coefficient corrects for non-ideal behavior and explains why a 1-molal electrolyte solution may exert a lower osmotic pressure than theoretically predicted.

Osomolality and osmolarity

Osmolality and osmolarity both describe solute concentration, but they differ in how this concentration is expressed.

• Osmolality refers to the number of osmoles per kilogram of solvent (Osmol/kg H₂O).
• Osmolarity refers to the number of osmoles per liter of solution (Osmol/L).

Because mass-based measurements are independent of temperature, osmolality is considered the more precise thermodynamic parameter. For this reason, clinical laboratories measure and report osmolality, even though the term osmolarity is frequently used in clinical literature.

Osmolality can be expressed as:

Osmolality = Osmol/(Kg H₂O) = ϕnC

Where:

C = molality (mol/kg H₂O

ϕ = osmotic coefficient

n = number of particles produced by dissociation

Glucose has an osmotic coefficient of approximately 1.0, while sodium chloride has an osmotic coefficient of about 0.93 at physiological concentrations, yielding an effective contribution of 1.86 osmoles per mmol of sodium. Ethanol has an osmotic coefficient of approximately 0.83.

In plasma, sodium and its accompanying anions (primarily chloride and bicarbonate) contribute most significantly to osmolality. Nonelectrolytes such as glucose and urea contribute less, while plasma proteins account for less than 0.5% of total plasma osmolality.

Determination of Plasma and Urine Osmolality

The measurement of plasma and urine osmolality is helpful in the diagnosis of electrolyte and acid-base imbalances. Na+, Cl-, glucose, and urea are the main osmotic substances in normal plasma; this predicted plasma osmolality is measured using the following analytical equation:

mOsmol/kg = 1.86[Na⁺(mmol/L)] + glucose [mmol/L]+urea[mmol/L]+9

Other osmotically active molecules in plasma, such as K⁺, Ca⁺⁺, and proteins, are represented by the 9 mOsmol/kg added to the previous equation. Notably, 1.86 is two times of osmotic coefficient of Na⁺, indicating the contributions of both Na⁺ and Cl^–. The + 9 mOsmol/kg parameter is excluded from certain versions of this equation.

mOsmol/kg = 2 x [Na⁺(mmol/L)] + glucose [mmol/L]+urea[mmol/L]

Plasma osmolality should be between 275 and 300 mOsmol/kg as a reference range. The level of hydration can have a significant impact on the osmolality of the urine. Osmotic concentrations, for instance, can drop to as low as 50 mOsm/kg H2O after excessive fluid intake, but concentrations of up to 1400 mOsm/kg H2O can be seen in people who strictly restrict their fluid intake. Values between 300 and 900 mOsm/kg H2O are generally observed in people who consume an average amount of fluids.

After 12 hours of fluid restriction, if a random urine sample has an osmolality of more than 600 mOsm/kg H2O, it is generally safe to infer that the renal concentrating ability is normal.

Measurement of osmolality

Any of the four colligative properties discussed previously—vapor pressure, boiling point, freezing point, and osmotic pressure—could theoretically be used as a basis for osmolality calculation. However, because of its simplicity, freezing point depression is the method of choice in diagnostic laboratories. In addition, unlike vapor pressure, freezing point depression is unaffected by temperature fluctuations.

Freezing Point Depression Osmometer

The components of a freezing point depression Osmometer include the following:

• A thermostatically controlled cooling bath or block maintained at -7⁰ C.
• A rapid stir mechanism to initiate freezing of the sample.
• A thermistor probe connected to a circuit to measure the temperature of the sample.
• A LED display that indicates the time course of the freezing curve and the final result.

The thermistor probe and stirring wire are centered in the sample, which is lowered into the bath and gently stirred to a temperature several degrees below freezing point (-7 ⁰C). The sample is lifted to a point above the liquid in the cooling bath when the LED display signals that proper super-cooling has occurred, and the wire stirrer is adjusted from a gentle rate of stir to a momentary vigorous amplitude, which starts freezing off the super-cooled solution. Just the slush level of freezing occurs, with around 2% to 3% of the solvent solidifying. The emitted heat of fusion warms the solution at first, then the temperature plateaus and stays stationary, suggesting the equilibrium temperature where both freezing and thawing of the solution are taking place.

Vapor Pressure Osmometer

A vapor pressure Osmometer is another kind of Osmometer. However, osmolality calculation with these instruments is specifically related to a decrease in dew point temperature of the pure solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes, rather than a change in vapor pressure (in millimeters of mercury). The inability of the vapor pressure technique to use any volatile solutes found in the serum in its calculation of overall osmolality is a significant clinical distinction between it and the freezing point depression Osmometer. Volatile substances such as ethanol, methanol, and isopropanol escape from the solution and increase the vapor pressure of the solvent (water) rather than lowering it. Because of this, using a vapor pressure osmometer to detect osmolal gaps in acid-base disturbances is impractical, and most clinical laboratories do not recommend using this type of osmometer.

Osmolal gap

An osmolal gap is a measurement of variance within the measured osmolality of a blood sample and the calculated osmolality established on electrolyte concentrations in the same sample. The normal osmolal gap range is usually between -10 and +10 mOsm/kg. Elevated osmolal gaps may suggest the presence of toxins or metabolic disorders such as hyperglycemia or the presence of alcohol or methanol.

Reference:

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics