1. What is Serum Osmolality?

Serum osmolality measures the concentration of dissolved particles in blood. It's important for evaluating fluid and electrolyte balance, and for diagnosing conditions like dehydration, diabetes insipidus, or toxin ingestion.

2. How Does the Calculator Work?

The calculator uses the serum osmolality equation:

Where:

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

Explanation: The equation accounts for the major osmotically active particles in serum. Sodium is doubled to account for accompanying anions, while glucose and BUN are converted to mmol/L.

3. Importance of Osmolality Calculation

Details: Osmolality helps assess hydration status, diagnose electrolyte disorders, evaluate for osmolar gaps (suggesting unmeasured osmoles like alcohols), and guide fluid therapy.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between osmolality and osmolarity?
A: Osmolality is measured per kg of solvent (more accurate), while osmolarity is per liter of solution. They're nearly equal for dilute solutions like serum.

Q2: What is an osmolar gap?
A: The difference between measured and calculated osmolality. A gap >10 suggests presence of unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Q3: When is osmolality most useful?
A: For evaluating altered mental status, suspected toxin ingestion, polyuria/polydipsia syndromes, and monitoring critically ill patients.

Q4: Does this account for all osmoles?
A: No, it calculates "effective" osmolality (tonicity). Other osmoles like potassium, calcium, and lipids are not included but contribute little to total osmolality.

Q5: How does hyperglycemia affect osmolality?
A: Glucose contributes to measured osmolality but doesn't cause fluid shifts between compartments (ineffective osmole) unless insulin is deficient.