1. What is Calculated Osmolality?

Calculated osmolality estimates the osmotic concentration of plasma based on its major solutes. It's used to evaluate fluid and electrolyte balance, and to identify osmolar gaps that may indicate unmeasured solutes.

2. How Does the Calculator Work?

The calculator uses the standard osmolality equation:

Where:

• \( Na^+ \) — Sodium concentration (mEq/L)
• \( K^+ \) — Potassium concentration (mEq/L)
• \( Glucose \) — Glucose concentration (mg/dL)
• \( BUN \) — Blood urea nitrogen (mg/dL)

Explanation: The equation accounts for the major osmotically active particles in plasma. Sodium and potassium are doubled to account for accompanying anions.

3. Clinical Significance

Details: Normal plasma osmolality is 275-295 mOsm/kg. Values outside this range may indicate dehydration, fluid overload, or the presence of unmeasured osmoles like ethanol or methanol.

4. Using the Calculator

Tips: Enter all values in their standard units. For accurate results, use recent laboratory measurements. The calculator assumes normal plasma composition.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between osmolality and osmolarity?
A: Osmolality is osmoles per kilogram of solvent (more accurate), while osmolarity is osmoles per liter of solution (temperature-dependent).

Q2: What is an osmolar gap?
A: The difference between measured and calculated osmolality. Gaps >10 mOsm/kg suggest unmeasured osmoles like alcohols or toxins.

Q3: Why is BUN included if it's freely permeable?
A: While urea crosses membranes freely, it still contributes to instantaneous osmotic pressure during measurement.

Q4: How does hyperglycemia affect osmolality?
A: Glucose at high concentrations significantly increases osmolality. For glucose >200 mg/dL, corrected sodium should be calculated.

Q5: When is direct osmolality measurement preferred?
A: When suspecting poisoning with volatile alcohols or other unmeasured solutes that create an osmolar gap.