1. What is Calculated Serum Osmolality?

Calculated serum osmolality is an estimate of the osmotic pressure of blood serum based on the concentrations of major solutes. The Mayo Clinic equation provides a reliable method for this calculation using sodium, glucose, and BUN values.

2. How Does the Calculator Work?

The calculator uses the Mayo Clinic 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 adjusted for their molecular weights.

3. Importance of Osmolality Calculation

Details: Serum osmolality is important for evaluating fluid and electrolyte balance, diagnosing osmolar disorders like hyperosmolar states, and assessing conditions like diabetes insipidus or syndrome of inappropriate antidiuretic hormone secretion (SIADH).

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 the normal range for serum osmolality?
A: Normal serum osmolality is typically 275-295 mOsm/kg. Values outside this range may indicate clinical abnormalities.

Q2: How does this differ from measured osmolality?
A: Measured osmolality includes all solutes, while calculated osmolality estimates based on major components. The osmolar gap (measured - calculated) can reveal unmeasured osmoles.

Q3: When is this calculation most useful?
A: Particularly valuable in evaluating patients with altered mental status, suspected alcohol intoxication, or disorders of water balance.

Q4: What substances does this equation not account for?
A: Doesn't include ethanol, methanol, ethylene glycol, or other exogenous osmoles that may contribute to measured osmolality.

Q5: How does hyperglycemia affect osmolality?
A: Glucose is included in the calculation. Severe hyperglycemia significantly increases serum osmolality, which can lead to hyperosmolar hyperglycemic state.