Osmolality (Serum)

Function

Serum Osmolality measures the concentration of dissolved particles in the blood, primarily sodium, glucose, and urea (BUN). It is a critical indicator of the body's water-electrolyte balance. The body maintains osmolality within a very narrow range through the action of Antidiuretic Hormone (ADH) and the thirst mechanism.

Why it is Ordered

This test is essential for evaluating disorders of hydration and sodium balance. Specific reasons for ordering include:

• Investigating hyponatremia (low blood sodium) to determine if it is 'true' hyponatremia.
• Evaluating suspected Diabetes Insipidus (where the body cannot conserve water).
• Assessing for the presence of 'unmeasured osmols' (the Osmolal Gap), which suggests poisoning by substances like ethanol, methanol, or ethylene glycol.
• Monitoring treatment for dehydration or hypernatremia.

Associated Conditions

• Dehydration: Causes increased concentration of particles.
• SIADH: Syndrome of Inappropriate Antidiuretic Hormone, leading to low serum osmolality.
• Diabetes Insipidus: Leads to high serum osmolality and low urine osmolality.
• Toxic Ingestion: Ingestion of glycols or alcohols significantly increases the osmolal gap.