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Serum Osmolality Calculator

Estimate the concentration of solutes in the blood to evaluate hydration and electrolyte status.

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Result

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Formula

(2 * Na) + (Glucose / 18) + (BUN / 2.8)

Estimates the number of particles per kilogram of solvent. Sodium, glucose, and urea are the primary contributors.

Worked Example

Na 140, Glucose 90, BUN 14 -> ~290 mOsm/kg.

Frequently Asked Questions

What is a normal serum osmolality?

Usually between 275 and 295 mOsm/kg.

What is the osmolar gap?

It is the difference between the measured osmolality (from a lab) and this calculated value. A gap > 10–12 suggests unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Does ethanol affect the gap?

Yes, ethanol significantly increases measured osmolality, creating a large osmolar gap.

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