1. What is Calculated Serum Osmolality?

Calculated serum osmolality is an estimate of the concentration of dissolved particles in blood. It's used to evaluate water balance and identify osmolar disorders like hyperosmolality or hypo-osmolality.

2. How Does the Calculator Work?

The calculator uses the serum osmolality equation:

Where:

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

Explanation: The equation accounts for the major osmotically active particles in serum. Sodium is doubled to account for accompanying anions.

3. Importance of Serum Osmolality

Details: Serum osmolality helps diagnose and monitor conditions like dehydration, diabetes insipidus, SIADH, and toxic alcohol ingestions (though this calculator excludes ethanol).

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 range is typically 275-295 mOsm/kg. Values outside this range indicate osmolar disorders.

Q2: How does this differ from measured osmolality?
A: Measured osmolality includes all solutes, while calculated estimates only major contributors. The osmolar gap is the difference between measured and calculated values.

Q3: When should ethanol be included in the calculation?
A: Ethanol should be added (ethanol/4.6) when evaluating possible alcohol intoxication or toxic alcohol ingestion.

Q4: What causes an increased osmolar gap?
A: Elevated gap suggests unmeasured osmoles like ethanol, methanol, ethylene glycol, or mannitol.

Q5: How does hyperglycemia affect osmolality?
A: Glucose contributes to osmolality but also causes water movement from cells, diluting sodium (translocational hyponatremia).