1. What Is Serum Osmolality?

Serum osmolality is a measure of the concentration of dissolved particles in blood plasma. It helps evaluate the body's water balance and the concentration of solutes like sodium, glucose, and urea.

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 is doubled because it's balanced by anions, while glucose and BUN are divided by their molecular weights.

3. Importance of Osmolality Calculation

Details: Serum osmolality is crucial for diagnosing and managing fluid and electrolyte disorders, evaluating dehydration, assessing renal function, and investigating suspected poisoning (e.g., ethanol, 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 serum osmolality range?
A: Normal range is typically 275-295 mOsm/kg. Values outside this range may indicate fluid or electrolyte imbalances.

Q2: What is the osmolar gap?
A: The difference between measured and calculated osmolality. A gap >10 suggests unmeasured osmoles (e.g., alcohols, toxins).

Q3: Why isn't potassium included in the equation?
A: Potassium's contribution is small and relatively constant, so it's typically omitted from the calculation.

Q4: How does hyperglycemia affect osmolality?
A: High glucose increases serum osmolality significantly. Each 18 mg/dL of glucose adds about 1 mOsm/kg.

Q5: When is direct osmolality measurement needed?
A: When suspecting poisoning with volatile alcohols or other unmeasured osmoles, or when the calculated osmolality doesn't match clinical findings.