Serum Osmolality/Osmolarity Calculator
Formula Used
Calculated Osmolality = (2 × Na⁺) + (Glucose ÷ 18) + (BUN ÷ 2.8)
What is Serum Osmolality?
Serum osmolality is a measure of the concentration of dissolved particles (solutes) in the blood. It represents the number of milliosmoles of solute per kilogram of serum (mOsm/kg) and is an important indicator of fluid and electrolyte balance in the body.
Calculation Formula
The calculated serum osmolality is estimated using the following formula:
Calculated Osmolality = (2 × Na⁺) + (Glucose ÷ 18) + (BUN ÷ 2.8)
Where:
- Na⁺ is serum sodium in mmol/L or mEq/L
- Glucose is in mg/dL (divide by 18 to convert to mmol/L)
- BUN (Blood Urea Nitrogen) is in mg/dL (divide by 2.8 to convert to mmol/L)
Osmolar Gap
The osmolar gap is the difference between the measured serum osmolality (determined by laboratory testing) and the calculated serum osmolality:
Osmolar Gap = Measured Osmolality − Calculated Osmolality
A normal osmolar gap is typically less than 10 mOsm/kg. An elevated osmolar gap suggests the presence of unmeasured osmotically active substances in the serum.
Clinical Significance
| Serum Osmolality | Interpretation |
|---|---|
| 275-295 mOsm/kg | Normal range |
| < 275 mOsm/kg | Hypoosmolality (e.g., SIADH, water intoxication) |
| > 295 mOsm/kg | Hyperosmolality (e.g., dehydration, hyperglycemia, mannitol therapy) |
| Osmolar Gap | Potential Causes |
|---|---|
| < 10 mOsm/kg | Normal |
| 10-15 mOsm/kg | Borderline elevated (may be normal in some laboratories) |
| > 15 mOsm/kg |
|
| > 25 mOsm/kg | Highly suggestive of toxic alcohol ingestion |
Clinical Applications
Serum osmolality and osmolar gap measurements are useful in several clinical scenarios:
- Toxic alcohol ingestion: Methanol, ethylene glycol, and isopropanol can cause a significant osmolar gap
- Diabetic emergencies: Helps assess severity of hyperglycemic states
- Electrolyte disorders: Useful in evaluating hyponatremia and hypernatremia
- Altered mental status: Can help identify causes of confusion or coma
- Renal disorders: Helps assess fluid status in kidney disease
Limitations
The calculated serum osmolality formula has several limitations:
- It does not account for all osmotically active substances in the blood
- Laboratory measurement techniques can vary, affecting the osmolar gap
- The formula assumes normal protein levels and acid-base status
- Different formulas exist with slight variations in coefficients
References
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162-174.
- Glasser L, Sternglanz PD, Combie J, Robinson A. Serum osmolality and its applicability to drug overdose. Am J Clin Pathol. 1973;60(5):695-699.
- Hoffman RS, Smilkstein MJ, Howland MA, Goldfrank LR. Osmol gaps revisited: normal values and limitations. J Toxicol Clin Toxicol. 1993;31(1):81-93.