serum osmolality
Summary
Serum osmolality measures the concentration of solute particles per kilogram of water in blood serum, normally 280-295 mOsm/kg. It reflects the body's water balance and is primarily determined by sodium, glucose, and urea concentrations. Critical for diagnosing and managing fluid/electrolyte disorders and altered mental status.
Detail
Serum osmolality quantifies the number of osmotically active particles dissolved in serum per kilogram of water. The major contributors are sodium (and its anions), glucose, and blood urea nitrogen (BUN), calculated as: 2 × [Na+] + [glucose]/18 + [BUN]/2.8. Normal values range from 280-295 mOsm/kg. Hyperosmolality (>295) can result from dehydration, hypernatremia, hyperglycemia, or ingestion of osmotically active substances (methanol, ethylene glycol). Hyposmolality (<280) typically indicates water excess or sodium depletion, seen in SIADH, psychogenic polydipsia, or certain medications. The osmolal gap (measured osmolality minus calculated osmolality) helps identify unmeasured osmoles like toxic alcohols. Serum osmolality drives water movement across cell membranes and is tightly regulated by ADH and the thirst mechanism. Rapid changes can cause cerebral edema or dehydration, making gradual correction essential in clinical management.
Sources
- Harrison's Principles of Internal Medicine
- Brenner & Rector's The Kidney
- First Aid for the USMLE Step 1
- UpToDate Clinical Decision Support
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