1. What is Serum Osmolality?

Serum osmolality is a measure of the number of dissolved particles in blood serum. It helps assess the body's water balance and electrolyte homeostasis. The calculated osmolality provides an estimate based on the major osmotically active substances in plasma.

2. How Does the Calculator Work?

The calculator uses the serum osmolality equation:

Where:

• \( Na^+ \) — Sodium concentration (mmol/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 converted from mg/dL to mmol/L.

3. Importance of Osmolality Calculation

Details: Serum osmolality is crucial for evaluating fluid and electrolyte disorders, diagnosing conditions like diabetes insipidus, and assessing the severity of alcohol intoxication. It's also used to monitor therapy for dehydration or overhydration.

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 expressed in mOsm/kg (milliosmoles per kilogram).

5. Frequently Asked Questions (FAQ)

Q1: What is the normal range for serum osmolality?
A: Normal serum osmolality ranges from 275 to 295 mOsm/kg. Values outside this range may indicate water imbalance or the presence of osmotically active substances.

Q2: What is the osmolar gap?
A: The osmolar gap is the difference between measured osmolality and calculated osmolality. A gap >10 mOsm/kg suggests the presence of unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Q3: Why isn't potassium included in the calculation?
A: Potassium's contribution is relatively small and stable, so it's typically omitted from the standard calculation. Some formulas may include it for more precise results.

Q4: How does hyperglycemia affect osmolality?
A: Elevated glucose increases serum osmolality. For very high glucose levels (>600 mg/dL), some formulas add a correction factor for the osmotic effect of glucose.

Q5: When is measured osmolality preferred?
A: Measured osmolality (by freezing point depression) is preferred when suspecting poisoning with alcohols or other unmeasured osmoles, or when the calculated value doesn't match clinical findings.