1. What is Plasma Osmolality?

Plasma osmolality measures the concentration of dissolved particles in blood plasma. It's important for assessing fluid and electrolyte balance, and for evaluating conditions like dehydration, diabetes insipidus, and toxic alcohol poisoning.

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 associated anions.

3. Clinical Significance

Details: Normal plasma osmolality ranges from 275-295 mOsm/kg. Values outside this range may indicate water imbalance, electrolyte disorders, or the presence of osmotically active substances like ethanol or methanol.

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 is measured per kilogram of solvent (more accurate), while osmolarity is per liter of solution. In clinical practice, the terms are often used interchangeably.

Q2: When is osmolality testing indicated?
A: Useful for evaluating altered mental status, seizures, polyuria/polydipsia, suspected toxic alcohol ingestion, and monitoring treatment for hyponatremia.

Q3: What is the osmolar gap?
A: The difference between measured and calculated osmolality. A gap >10 suggests unmeasured osmoles (e.g., ethanol, methanol, ethylene glycol).

Q4: Does this equation account for other osmoles?
A: No, it only includes major contributors. Other substances (e.g., ethanol, mannitol) will increase measured osmolality but aren't included in this calculation.

Q5: How does hyperglycemia affect osmolality?
A: Glucose contributes to osmolality but also causes water movement from cells to plasma, diluting sodium ("corrected sodium" should be calculated in hyperglycemia).