1. What is Calculated Serum Osmolality?

Calculated serum osmolality is an estimate of the osmotic pressure of blood based on the concentrations of major solutes. It's used to evaluate fluid and electrolyte balance, and to detect osmolar gaps that may indicate the presence of unmeasured osmoles like ethanol or methanol.

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 contributors to serum osmolality. Sodium (with its accompanying anions) is doubled to account for both cations and anions. Glucose and BUN are divided by factors to convert mg/dL to mmol/L equivalents.

3. Clinical Importance

Details: Serum osmolality is important for evaluating disorders of water balance (hyponatremia, hypernatremia), detecting toxin ingestion (alcohols, glycols), and assessing dehydration or fluid overload.

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). Normal serum osmolality ranges from 275-295 mOsm/kg.

5. Frequently Asked Questions (FAQ)

Q1: What is the osmolar gap?
A: The difference between measured and calculated osmolality. A gap >10 mOsm/kg suggests the presence of unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Q2: Why isn't potassium included in the equation?
A: Potassium's contribution is relatively small and stable, so it's typically omitted from this calculation.

Q3: How does hyperglycemia affect osmolality?
A: Glucose contributes to osmolality, but in hyperglycemia, water shifts from intracellular to extracellular space, diluting sodium (pseudohyponatremia).

Q4: When is measured osmolality preferred?
A: When evaluating for toxic alcohol ingestion or when an osmolar gap is suspected.

Q5: What conditions increase calculated osmolality?
A: Dehydration, hypernatremia, hyperglycemia, uremia, and mannitol administration.