Osmolar Gap Calculator – Serum Osmolality Assessment
Calculate serum osmolar gap for toxicology assessment
For informational purposes only. Osmolar gap calculations should be interpreted by qualified healthcare professionals in clinical context.
Table of Contents
How to Use
- Enter measured serum osmolality (mOsm/kg)
- Enter serum sodium level (mEq/L)
- Enter blood glucose level (mg/dL)
- Enter blood urea nitrogen - BUN (mg/dL)
- Click calculate to see osmolar gap and interpretation
What is the Osmolar Gap?
The osmolar gap is the difference between measured serum osmolality and calculated serum osmolality. It helps identify unmeasured osmotically active substances in the blood, particularly in toxicology and metabolic assessments.
Calculated osmolality is based on the major osmotically active particles in serum: sodium (and its anions), glucose, and urea (measured as BUN). The formula is: 2 × Na + (Glucose/18) + (BUN/2.8)
Interpreting Osmolar Gap Values
| Osmolar Gap | Category | Clinical Significance |
|---|---|---|
| < 10 mOsm/kg | Normal | No significant unmeasured osmoles |
| 10-25 mOsm/kg | Elevated | May indicate presence of unmeasured substances, requires investigation |
| > 25 mOsm/kg | Highly Elevated | Strongly suggests toxic ingestion or significant metabolic disturbance |
Causes of Elevated Osmolar Gap
An elevated osmolar gap can result from various conditions:
- Toxic alcohols: Methanol, ethylene glycol, isopropanol, diethylene glycol
- Ketones: Diabetic ketoacidosis, alcoholic ketoacidosis
- Lactic acid: Severe lactic acidosis
- Mannitol or other exogenous osmoles
- Ethanol ingestion (most common benign cause)
- Laboratory error or analytical interference
- Chronic kidney disease (mild elevation)
- Hyperlipidemia or hyperproteinemia (pseudohyponatremia)
Toxic Alcohol Poisoning
Toxic alcohols are a critical cause of elevated osmolar gap:
- Methanol: Found in antifreeze, windshield washer fluid, fuel; metabolizes to formic acid causing severe acidosis and blindness
- Ethylene glycol: Found in antifreeze; metabolizes to glycolic and oxalic acid causing acidosis and acute kidney injury
- Isopropanol: Found in rubbing alcohol; metabolizes to acetone; causes CNS depression but not acidosis
- Diethylene glycol: Found in some solvents and contaminated medications; causes kidney and liver damage
Note: As toxic alcohols are metabolized, the osmolar gap may decrease while anion gap metabolic acidosis develops. Early detection and treatment are critical.
Clinical Evaluation and Management
When an elevated osmolar gap is detected:
- Obtain detailed history including potential toxic exposures
- Check anion gap and arterial blood gas for metabolic acidosis
- Measure ethanol level (common cause of elevated osmolar gap)
- Consider toxic alcohol levels if history or presentation suggests ingestion
- Assess for ketones (beta-hydroxybutyrate, acetone)
- Review medications and recent procedures (mannitol, contrast)
- Verify laboratory results and repeat if measurement error suspected
- Consider immediate treatment with fomepizole or ethanol for suspected toxic alcohol ingestion
- Consult toxicology or nephrology as needed
Limitations and Considerations
Important limitations of the osmolar gap:
- Normal osmolar gap does not exclude toxic alcohol poisoning if significant time has passed (alcohols may be metabolized)
- Different laboratories may have slightly different normal ranges
- Calculation assumes normal sodium, glucose, and BUN contributions
- Ethanol is a common cause of elevated osmolar gap but is not toxic in the same way as other alcohols
- Must be interpreted in clinical context with other laboratory findings
- Mild elevations may be seen in chronic kidney disease without toxicity
Frequently Asked Questions
- What is a normal osmolar gap?
- A normal osmolar gap is typically less than 10 mOsm/kg. Values between 10-25 mOsm/kg are considered elevated and may warrant investigation, while values above 25 mOsm/kg are highly elevated and strongly suggest the presence of unmeasured osmoles.
- What does an elevated osmolar gap indicate?
- An elevated osmolar gap indicates the presence of unmeasured osmotically active substances in the blood. Common causes include toxic alcohols (methanol, ethylene glycol), ethanol, ketones, or laboratory error. Clinical correlation is essential for proper interpretation.
- Can toxic alcohol poisoning be present with a normal osmolar gap?
- Yes. If significant time has passed since ingestion, toxic alcohols may be metabolized into toxic acids, causing the osmolar gap to normalize while developing an anion gap metabolic acidosis. Early detection is critical.
- What is the difference between osmolar gap and anion gap?
- Osmolar gap measures the difference between measured and calculated osmolality, identifying unmeasured osmoles. Anion gap measures the difference between measured cations and anions, identifying unmeasured anions (like lactate or ketones). Both are used in toxicology and metabolic assessments but measure different things.
- When should I be concerned about an elevated osmolar gap?
- Be concerned if the osmolar gap is >10 mOsm/kg with symptoms of toxicity, altered mental status, unexplained metabolic acidosis, or history of possible toxic ingestion. Immediate medical evaluation is needed for suspected toxic alcohol poisoning.
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