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Serum Osmolality

Used to assess the concentration of solutes in blood and helps evaluate fluid balance, electrolyte disturbances, and toxin ingestion

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Serum Osmolality

Used to assess the concentration of solutes in blood and helps evaluate fluid balance, electrolyte disturbances, and toxin ingestion

Serum Sodium

Normal range: 135-145 mEq/L

BUN

Normal range: 6-20 mg/dL

Serum Glucose

Normal range: 70-140 mg/dL

Ethanol

Optional (Normal range: 0 mg/dL)

Serum Osmolality — —

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Instructions

To calculate, input serum sodium, glucose, and blood urea nitrogen (BUN) into the standard formula. Direct measurement can also be done via laboratory osmometry. Always compare the measured and calculated values to check for an osmolal gap, which can indicate the presence of unmeasured solutes such as toxic alcohols.

Overview

When to use

Why use

Evidences

Interpretation

Serum osmolarity = (2 × Na + (BUN / 2.8) + (glucose / 18))

Comparative analyses indicate the simple formula 2×Na + glucose/18 + BUN/2.8 performs as well as more complex equations for plasma osmolality estimation across general populations, supporting its use at the bedside

https://www.sciencedirect.com/science/article/abs/pii/S000991201500082X

Osmolal gap = measured osmolality (osmometer) − calculated osmolality. A normal gap is typically <10 mOsm/kg; larger gaps suggest unmeasured osmoles (e.g., toxic alcohols, mannitol, glycols, ketones), though lab and patient factors can affect the gap

https://pmc.ncbi.nlm.nih.gov/articles/PMC10166247/

Emergency/toxicology literature emphasizes that all alcohols, including ethanol, alter the osmolal gap; large gaps (>20) warrant evaluation for toxic alcohol ingestion along with anion gap and acid–base status

https://www.sciencedirect.com/science/article/abs/pii/S0196064424013039

During hyperosmolar therapy (mannitol or hypertonic saline), calculated osmolality may deviate from measured values; several studies show variable agreement depending on the formula used, advocating direct osmometry or validated formulas and attention to the osmolal gap when monitoring therapy

https://pubmed.ncbi.nlm.nih.gov/31983046/

Overview

When to use

Why use

Evidences

Serum osmolality reflects the number of dissolved particles in plasma and is a key indicator of the body’s hydration and electrolyte status. It plays a critical role in maintaining osmotic equilibrium between intracellular and extracellular compartments, guided primarily by sodium, glucose, and urea levels.

The osmolal gap is the difference between measured and calculated osmolality. A gap >10–15 mOsm/kg typically indicates unmeasured solutes, most importantly toxic alcohols.

Clinically, serum osmolality is used in the evaluation of hyponatremia, hypernatremia, suspected poisoning, and altered mental status. It helps guide fluid therapy and the management of electrolyte imbalances. Its utility spans emergency medicine, nephrology, toxicology, and endocrinology.