1. What is Plasma Osmolality?

Plasma osmolality measures the concentration of dissolved particles in blood plasma. It's crucial for maintaining proper fluid balance between intracellular and extracellular compartments. The normal range is 275-295 mOsm/kg.

2. How Does the Calculator Work?

The calculator uses the standard plasma osmolality equation:

Where:

• \( Na \) — Serum sodium (mmol/L)
• \( Glucose \) — Serum glucose (mg/dL)
• \( BUN \) — Blood urea nitrogen (mg/dL)

Explanation: The equation accounts for the major osmotically active particles in plasma. Sodium is doubled to account for associated anions.

3. Clinical Importance

Details: Plasma osmolality is used to evaluate fluid and electrolyte balance, diagnose disorders like diabetes insipidus, and assess toxin ingestion (e.g., ethanol, 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 measures particles per kg of solvent (more accurate), while osmolarity measures per liter of solution. They're nearly equal in dilute solutions like plasma.

Q2: When is measured osmolality needed?
A: Measured osmolality (by freezing point depression) is needed when suspecting unmeasured osmoles (e.g., alcohols, toxins) or when the osmolar gap is important.

Q3: What causes high osmolality?
A: Dehydration, hypernatremia, uncontrolled diabetes (hyperglycemia), uremia, or toxic alcohol ingestion.

Q4: What causes low osmolality?
A: Hyponatremia, SIADH, or overhydration.

Q5: How does ethanol affect osmolality?
A: Ethanol increases measured osmolality but isn't included in calculated osmolality, creating an osmolar gap.