1. What Is Osmolality?

Osmolality is a measure of the number of osmoles of solute per kilogram of solvent. In clinical medicine, serum osmolality is important for evaluating fluid and electrolyte balance, and for detecting conditions like dehydration, diabetes insipidus, and toxic alcohol ingestion.

2. How Does the Calculator Work?

The calculator uses the following equation:

Where:

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

Explanation: The equation accounts for the major contributors to serum osmolality. Sodium is doubled because it's accompanied by anions (mainly chloride). Glucose and BUN are divided by conversion factors to account for their molecular weights.

3. Clinical Importance of Osmolality

Details: Osmolality is crucial for diagnosing and managing conditions like hyponatremia, hypernatremia, and toxic alcohol ingestions. The osmolar gap (measured osmolality - calculated osmolality) can indicate the presence of unmeasured osmoles like ethanol, methanol, or ethylene glycol.

4. Using the Calculator

Tips: Enter sodium in mmol/L, glucose and BUN in mg/dL. All values must be valid (sodium > 0, glucose and BUN ≥ 0). The result is in mOsm/kg.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal serum osmolality?
A: Normal serum osmolality ranges from 275-295 mOsm/kg. Values outside this range may indicate pathology.

Q2: What is the osmolar gap?
A: The difference between measured and calculated osmolality. Normal gap is <10 mOsm/kg. Elevated gaps suggest presence of unmeasured osmoles.

Q3: Why isn't potassium included in the equation?
A: Potassium concentration is much lower than sodium and its contribution is negligible in most cases.

Q4: How does ethanol affect osmolality?
A: Ethanol contributes to measured osmolality but isn't included in this calculation. Its presence increases the osmolar gap.

Q5: When is direct osmolality measurement needed?
A: When toxic alcohol ingestion is suspected or when the clinical picture doesn't match the calculated osmolality.