Serum osmolality

Serum osmolality is a measure of the solute concentration in the blood. It is an important indicator of the body's hydration status and can provide critical information about the balance of water and solutes in the body. Serum osmolality is typically measured in milliosmoles per kilogram (mOsm/kg) of water. This measurement reflects the concentration of particles such as sodium, chloride, bicarbonate, glucose, and urea in the serum.

Overview

The normal range for serum osmolality is approximately 275 to 295 mOsm/kg. Values outside of this range can indicate various medical conditions. High serum osmolality (hyperosmolality) suggests dehydration or an excess of solutes in the blood, which can be caused by conditions such as diabetes insipidus, hypernatremia, or hyperglycemia. Conversely, low serum osmolality (hyposmolality) indicates overhydration or a deficiency of solutes, which can be seen in conditions like hyponatremia, syndrome of inappropriate antidiuretic hormone secretion (SIADH), or excessive water intake.

Measurement

Serum osmolality is measured using a laboratory technique called osmometry. There are two main types of osmometry: freezing point depression osmometry and vapor pressure osmometry. Freezing point depression osmometry is more commonly used for measuring serum osmolality. This method is based on the principle that the freezing point of a solution is lower than that of pure water, and the decrease in freezing point is proportional to the number of dissolved particles in the solution.

Clinical Significance

The measurement of serum osmolality is crucial in the diagnosis and management of various electrolyte and water balance disorders. It is particularly useful in the evaluation of patients with altered mental status, as changes in osmolality can affect brain function. Additionally, serum osmolality is used to calculate the osmolar gap, which is the difference between the measured serum osmolality and the calculated osmolality. An increased osmolar gap can indicate the presence of unmeasured osmoles, such as toxins or other substances, in the blood.

Osmolar Gap

The osmolar gap is calculated using the formula: \[ \text{Osmolar Gap} = \text{Measured Serum Osmolality} - \text{Calculated Serum Osmolality} \] The calculated serum osmolality can be estimated using the formula: \[ \text{Calculated Serum Osmolality} = 2 \times [\text{Na}^+] + [\text{Glucose}]/18 + [\text{BUN}]/2.8 \] where Na+ is the serum sodium concentration (mmol/L), glucose is the serum glucose concentration (mg/dL), and BUN is the blood urea nitrogen concentration (mg/dL).

Conditions Associated with Abnormal Serum Osmolality

• Hyperosmolality:

 * Diabetes insipidus
 * Hypernatremia
 * Hyperglycemia

• Hyposmolality:

 * Hyponatremia
 * Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
 * Excessive water intake

See Also

• Electrolyte imbalance
• Water-electrolyte imbalance
• Diabetes insipidus
• Hyponatremia