Serum Osmolar Gap Calculator
Calculate the Serum Osmolar Gap using measured serum osmolality, sodium, glucose, BUN, and ethanol. This calculator provides the calculated osmolality, osmolar gap, and clinical interpretation.
Calculation Result
What Is the Serum Osmolar Gap?
The serum osmolar gap is the difference between measured serum osmolality and calculated serum osmolality. It can provide supportive information when unmeasured osmotically active substances are suspected in the blood.
Formula
Calculated Serum Osmolality:
(2 × Sodium) + (Glucose ÷ 18) + (BUN ÷ 2.8)
When ethanol is included:
Calculated Osmolality = (2 × Sodium) + (Glucose ÷ 18) + (BUN ÷ 2.8) + (Ethanol ÷ 4.6)
Osmolar Gap:
Measured Osmolality − Calculated Osmolality
Osmolar Gap Interpretation
| Osmolar Gap | General Interpretation |
|---|---|
| -10 to +10 mOsm/kg | Generally within expected range |
| >10 to 20 mOsm/kg | Mildly elevated |
| >20 to 30 mOsm/kg | Elevated; consider unmeasured osmoles |
| >30 mOsm/kg | Markedly elevated; requires clinical evaluation |
Clinical Use
The osmolar gap can be useful when evaluating unexplained metabolic abnormalities and suspected exposure to osmotically active substances. It may be considered in the assessment of suspected toxic alcohol ingestion, unexplained high anion gap metabolic acidosis, and other clinical situations where an elevated concentration of unmeasured osmoles is suspected.
Unmeasured Osmoles
An increased osmolar gap can occur when substances that contribute to measured serum osmolality are not adequately represented in the calculated formula. Examples may include toxic alcohols and other osmotically active substances.
- Methanol
- Ethylene glycol
- Isopropanol
- Other unmeasured osmotically active substances
Frequently Asked Questions
What is a normal osmolar gap?
A commonly used general reference is approximately -10 to +10 mOsm/kg, although the expected range may vary depending on the laboratory, measurement method, and calculation used.
What causes an elevated osmolar gap?
An elevated osmolar gap may occur when unmeasured osmotically active substances are present. Toxic alcohols are important clinical considerations, but an elevated gap is not specific for toxic alcohol poisoning.
Can the osmolar gap diagnose toxic alcohol poisoning?
No. The osmolar gap is a supportive laboratory finding and should not be used alone to diagnose toxic alcohol poisoning. Clinical findings, acid-base status, laboratory testing, and toxicology investigations should also be considered.
Why is ethanol included in the calculation?
Ethanol contributes to measured serum osmolality. When ethanol concentration is known, its estimated osmotic contribution can be included in the calculated osmolality.
Clinical Applications
- Evaluation of suspected toxic alcohol exposure
- Assessment of unexplained high anion gap metabolic acidosis
- Evaluation of unexplained elevated serum osmolality
- Assessment of unmeasured osmoles
- Critical care laboratory evaluation
Limitations
- The osmolar gap is an estimate and depends on the formula used.
- Different laboratories may use different reference intervals.
- An elevated osmolar gap is not specific for toxic alcohol poisoning.
- Clinical findings and additional laboratory investigations are required for diagnosis.
Disclaimer
This calculator is intended for educational and laboratory reference purposes only. It should not replace professional medical advice, diagnosis, or treatment. Clinical decisions should be based on the complete clinical picture, laboratory findings, and applicable clinical guidelines.
References
- Tietz Fundamentals of Clinical Chemistry and Molecular Diagnostics.
- Henry's Clinical Diagnosis and Management by Laboratory Methods.
- American Association for Clinical Chemistry (AACC).
