1. What is the Osmolality Formula?

The osmolality formula estimates serum osmolality using sodium, glucose, and blood urea nitrogen (BUN) levels. It's used to evaluate the body's water-electrolyte balance and to detect conditions like dehydration, diabetes insipidus, or toxin ingestion.

2. How Does the Calculator Work?

The calculator uses the 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 substances in blood. Sodium is doubled to account for accompanying anions, while glucose and BUN are divided by conversion factors to express them in mmol/L.

3. Importance of Osmolality Calculation

Details: Osmolality helps assess fluid balance, diagnose electrolyte disorders, evaluate renal concentrating ability, and detect osmolar gaps that 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 osmolality range?
A: Normal serum osmolality 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 the presence of unmeasured osmotically active substances.

Q3: When is this calculation most useful?
A: In evaluating hyponatremia, polyuria/polydipsia syndromes, suspected toxin ingestion, and monitoring patients receiving hypertonic solutions.

Q4: Does this account for all osmotically active substances?
A: No, it only includes the major contributors. Other substances like ethanol, mannitol, or glycine will increase measured osmolality.

Q5: How does this differ from osmolarity?
A: Osmolality is measured per kilogram of solvent (more accurate), while osmolarity is per liter of solution. They're nearly equal for dilute solutions like plasma.