1. What is Osmolality?

Osmolality is a measure of the number of dissolved particles in a fluid. It's an important indicator of the body's water-electrolyte balance and is typically measured in milliosmoles per kilogram (mOsm/kg).

2. Clinical Significance

Osmolality is crucial for evaluating:

• Hydration status
• Electrolyte imbalances
• Renal function
• Diabetes insipidus
• Syndrome of inappropriate antidiuretic hormone (SIADH)

3. Normal Range

Normal serum osmolality: 275-295 mOsm/kg
Values below 275: May indicate overhydration or hyponatremia
Values above 295: May indicate dehydration or hypernatremia

4. Interpretation

A value of 278 mOsm/kg is within the normal range, suggesting proper water-electrolyte balance. No further action is typically needed based solely on this result.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between osmolality and osmolarity?
A: Osmolality is measured per kilogram of solvent (more accurate), while osmolarity is per liter of solution.

Q2: How is osmolality measured?
A: Typically by freezing point depression osmometry, though vapor pressure and dew point methods also exist.

Q3: When should osmolality be tested?
A: When evaluating fluid/electrolyte disorders, altered mental status, or suspected poisoning.

Q4: What affects osmolality?
A: Primarily sodium, glucose, and urea levels, though other solutes contribute.

Q5: Can I calculate osmolality?
A: Yes, estimated serum osmolality = 2×Na + glucose/18 + BUN/2.8 (all in mg/dL).