Osmolal Gap
Understanding the Osmolal Gap
The Osmolal Gap is a valuable diagnostic tool that helps detect the presence of unmeasured osmotically active substances in the blood. It's calculated by finding the difference between measured serum osmolality and calculated serum osmolality.
What is the Osmolal Gap?
The Osmolal Gap represents the difference between:
- Measured serum osmolality (determined by laboratory testing using freezing point depression method)
- Calculated serum osmolality (estimated using one of several validated equations)
Normal Values
- Normal range: -10 to +10 mOsm/kg
- Values outside this range suggest the presence of unmeasured osmotically active substances
- Results should be interpreted within clinical context and laboratory-specific reference ranges
Clinical Significance
Elevated Osmolal Gap (> 10 mOsm/kg)
May indicate presence of:
- Toxic alcohols (methanol, ethylene glycol)
- Ethanol intoxication
- Mannitol administration
- Isopropyl alcohol
- Other unmeasured osmoles
Normal or Low Osmolal Gap
Usually indicates:
- No significant unmeasured osmoles
- Normal metabolic state
- Proper laboratory measurement
Common Causes of Elevated Osmolal Gap
-
Toxic Ingestions
- Methanol
- Ethylene glycol
- Isopropyl alcohol
- Ethanol
-
Medical Treatments
- Mannitol administration
- Glycine irrigation
- Radiocontrast agents
-
Metabolic Conditions
- Diabetic ketoacidosis
- Alcoholic ketoacidosis
- Lactic acidosis
When to Calculate Osmolal Gap
Consider measuring when:
- Suspected toxic alcohol ingestion
- Unexplained metabolic acidosis
- Altered mental status
- Unexplained acute kidney injury
- Treatment monitoring for toxic alcohol ingestion
Important Considerations
- The calculation assumes no significant amounts of alcohols or other osmotically active substances are present
- Different equations may give slightly different results - consistency in equation choice is important
- Laboratory-measured osmolality remains the gold standard
Clinical Applications
The Osmolal Gap is particularly useful in:
- Emergency medicine
- Toxicology
- Critical care
- Metabolic disorder evaluation
- Monitoring treatment response
Start calculating now to better understand your patient's osmolar status!
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