1. What is Plasma Osmolality?

Plasma osmolality measures the concentration of dissolved particles in blood plasma. It's an important indicator of water balance and kidney function, helping to evaluate hydration status and diagnose conditions like dehydration, diabetes insipidus, or 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 because it's accompanied by anions (mainly chloride and bicarbonate).

3. Importance of Osmolality Calculation

Details: Plasma osmolality is crucial for assessing fluid and electrolyte balance, diagnosing hyponatremia, evaluating renal concentrating ability, and detecting osmolar gaps which may indicate 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). Normal plasma osmolality ranges from 275-295 mOsm/kg.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal osmolality range?
A: Normal plasma osmolality is typically 275-295 mOsm/kg. Values outside this range may indicate water imbalance.

Q2: 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).

Q3: Why isn't potassium included in the equation?
A: Potassium's contribution is small relative to sodium and is typically accounted for within the "2 × Na" term.

Q4: How does this differ from urine osmolality?
A: Urine osmolality measures kidney concentrating ability, while plasma osmolality reflects overall body water balance.

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