1. What is Plasma Osmolality?

Plasma osmolality measures the concentration of dissolved particles in blood plasma. It's important for assessing fluid balance, electrolyte disorders, and conditions like diabetes insipidus or syndrome of inappropriate antidiuretic hormone (SIADH).

2. How Does the Calculator Work?

The calculator uses the plasma osmolality equation:

Where:

• \( Na \) — Serum sodium concentration (mmol/L)
• \( Glucose \) — Blood 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 helps evaluate hydration status, diagnose electrolyte imbalances, and assess conditions affecting water balance. It's particularly useful in evaluating hyponatremia and hypernatremia.

4. Using the Calculator

Tips: Enter sodium in mmol/L, glucose in mg/dL, and BUN in mg/dL. All values must be valid positive numbers. Normal plasma osmolality ranges from 275-295 mOsm/kg.

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 (affected by temperature and pressure).

Q2: What is the osmolar gap?
A: The difference between measured and calculated osmolality. An elevated gap (>10 mOsm/kg) suggests unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Q3: When is this calculation most useful?
A: In evaluating hyponatremia (to distinguish between hypotonic, isotonic, and hypertonic hyponatremia) and in toxic alcohol ingestions.

Q4: Does this account for all osmotically active particles?
A: No, it's an estimate. Other substances like potassium, calcium, and magnesium also contribute but are omitted for simplicity.

Q5: How does hyperglycemia affect osmolality?
A: Glucose contributes to osmolality, but in hyperglycemia, water shifts from intracellular to extracellular space, diluting sodium (translocational hyponatremia).