1. What is Plasma Osmolality?

Plasma osmolality measures the concentration of dissolved particles in blood plasma. It's important for evaluating fluid and electrolyte balance, diagnosing disorders of water metabolism, and assessing dehydration or overhydration.

2. How Does the Calculator Work?

The calculator uses the standard plasma osmolality equation:

Where:

• \( Na \) — Sodium concentration (mmol/L)
• \( Glucose \) — Glucose concentration (mg/dL)
• \( BUN \) — Blood urea nitrogen concentration (mg/dL)

Explanation: The equation accounts for the major osmotically active particles in plasma. Sodium is doubled to account for accompanying anions, while glucose and BUN are divided by conversion factors.

3. Clinical Importance

Details: Normal plasma osmolality ranges from 275-295 mOsm/kg. Values outside this range may indicate water imbalance, diabetes insipidus, SIADH, or toxic alcohol ingestion.

4. Using the Calculator

Tips: Enter sodium in mmol/L, glucose and BUN in mg/dL. All values must be valid (sodium > 0, glucose and BUN ≥ 0).

5. Frequently Asked Questions (FAQ)

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

Q2: What's the osmolar gap?
A: The difference between measured and calculated osmolality. A gap >10 suggests unmeasured osmoles (e.g., ethanol, methanol, ethylene glycol).

Q3: Why isn't potassium included?
A: Potassium's extracellular concentration is relatively small and stable, so its contribution is negligible in this calculation.

Q4: How does hyperglycemia affect osmolality?
A: Glucose contributes to osmolality but also causes water shifts from intracellular to extracellular space, diluting sodium.

Q5: When is direct osmolality measurement needed?
A: When toxic alcohol ingestion is suspected or when the calculated osmolality doesn't match clinical findings.