1. What is Serum Osmolality?

Serum osmolality is a measure of the number of dissolved particles per kilogram of serum. It helps evaluate the body's water balance and is used to diagnose conditions like dehydration, diabetes insipidus, and toxic alcohol poisoning.

2. How Does the Calculator Work?

The calculator uses the MDCalc serum osmolality formula:

Where:

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

Explanation: The formula accounts for the major osmotically active particles in serum: sodium (and its accompanying anions), glucose, and urea.

3. Importance of Osmolality Calculation

Details: Serum osmolality is crucial for evaluating fluid and electrolyte disorders, assessing hydration status, and detecting osmolar gaps which may indicate the presence of unmeasured osmoles like ethanol or methanol.

4. Using the Calculator

Tips: Enter sodium in mEq/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 is a normal serum osmolality range?
A: Normal range is typically 275-295 mOsm/kg. Values outside this range may indicate fluid or electrolyte disorders.

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: Why is sodium multiplied by 2 in the formula?
A: Sodium is multiplied by 2 to account for its accompanying anions (mainly chloride and bicarbonate) that maintain electroneutrality.

Q4: When is this calculation most useful?
A: Particularly useful in evaluating altered mental status, suspected toxic alcohol ingestion, and disorders of water balance.

Q5: Are there limitations to this equation?
A: It doesn't account for other osmotically active substances like ethanol, mannitol, or glycine which may be present in certain clinical situations.