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M04.03.04 Osmolar Gap
The osmolar gap is an important clinical calculation used to identify unmeasured osmotically active substances in the blood. It is the difference between the measured osmolality and the estimated osmolality. The osmolar gap is used in the assessment of metabolic disorders, particularly in cases of suspected toxic ingestion.
ECF Estimated Osmolality Formula: ECF estimated osmolality = 2 × ( Na + ) mEq/L + Glucose(mg/dL)/ 18 + Urea(mg/dL)/ 2.8Calculation of Osmolar Gap
ECF Estimated Osmolality Formula: 2×(Na+)mEq/L+(Glucose mg/dL)/18+(Urea mg/dL)/2.8
Components of the Formula
- Sodium (Na⁺): Sodium is the most abundant cation in the extracellular space. It is multiplied by 2 because, for every sodium ion (positive charge), there is typically a corresponding anion (negative charge), such as chloride or bicarbonate.
- Glucose: Glucose contributes to the osmolality and is converted from its typical unit (mg/dL) into mOsm/kg by dividing by 18.
- Urea (Blood Urea Nitrogen – BUN): Urea also contributes to osmolality and is similarly converted from mg/dL into mOsm/kg by dividing by 2.8.
Table: Osmolar Gap Components and Conversion Factors
| Component | Typical Unit | Conversion Factor | Contribution to Osmolality (mOsm/kg) |
|---|---|---|---|
| Sodium (Na⁺) | mEq/L | ×2 | Major extracellular cation |
| Glucose | mg/dL | ÷18 | Carbohydrate affecting osmolality |
| Urea (BUN) | mg/dL | ÷2.8 | Nitrogenous waste affecting osmolality |
Interpretation of the Osmolar Gap
- Normal Osmolar Gap: ≤ 15 mOsm/kg
- Elevated Osmolar Gap: An osmolar gap greater than 15 indicates the presence of unmeasured osmoles. This can occur in cases of toxic ingestion or metabolic disorders.
Common Causes of Elevated Osmolar Gap
Some of the more frequent causes of an elevated osmolar gap include:
- Ethanol
- Methanol
- Ethylene glycol
- Acetone
- Mannitol
An elevated osmolar gap, especially in an inebriated patient, may indicate poisoning with one of these substances, particularly ethanol or methanol.
Points to Remember
- Na⁺ is the most abundant extracellular cation, and its concentration plays a central role in osmolality.
- Glucose and urea are significant contributors to osmolality but require conversion from their usual units.
- An osmolar gap > 15 suggests the presence of unmeasured substances, commonly seen in cases of poisoning.
- The osmolar gap is a useful diagnostic tool in emergency settings to identify potential toxins in the bloodstream.
Bibliography
- Adrogué, H.J., & Madias, N.E. (2000). Hypernatremia. New England Journal of Medicine, 342(20), 1493-1499.
- Krasowski, M.D., & Wilcoxon, R.M. (2017). Laboratory Assessment of Osmolality and Osmolar Gap. Clinical Chemistry, 63(5), 1078-1080.