Serum Osmolality Calculator 2025: Osmolar Gap, Toxic Alcohols, DKA
Serum osmolality reflects solute concentration in blood. The osmolar gap detects unmeasured osmotically active substances like methanol, ethylene glycol, and mannitol. Our Serum Osmolality Calculator computes measured, calculated osmolality, and gap using standard formulas.
Why Measure Osmolality?
Used in altered mental status, anion gap acidosis, and suspected ingestion. Normal: 275–295 mOsm/kg.
Key Formulas
1. Calculated Osmolality (2 × Na + Glucose/18 + BUN/2.8)
- Units: Na (mEq/L), Glucose (mg/dL), BUN (mg/dL)
- SI: 2 × Na + Glucose + Urea
2. Osmolar Gap = Measured − Calculated
- Normal: <10 mOsm/kg
- Elevated: >10 → toxic alcohols, ketoacidosis
Interpretation Table
| Gap | Cause | Action |
|---|---|---|
| <10 | Normal | Reassure |
| 10–20 | Ethanol, mild DKA | Check ethanol |
| >20 | Methanol, ethylene glycol | Fomepizole, dialysis |
Clinical Scenarios
- Toxic Alcohols: Methanol (gap + acidosis), Ethylene Glycol (calcium oxalate)
- DKA/HHS: Gap from ketones, dehydration
- Mannitol: Post-neurosurgery
- Hyponatremia: SIADH vs hypovolemic
Step-by-Step Example
Na 135, Glucose 180, BUN 28, Measured 320:
- Calc = 2×135 + 180/18 + 28/2.8 = 270 + 10 + 10 = 290
- Gap = 320 – 290 = 30 → suspect methanol
Best Practices
- Send measured osmolality with basic metabolic panel
- Correct for ethanol: subtract ethanol/3.7 from gap
- Repeat in 6h if suspicion high
- Pair with anion gap
- Use in coma of unknown origin
Advanced Topics
- Ethanol: Contributes 1 mOsm per 4.6 mg/dL
- Isopropanol: High gap, no acidosis
- Propylene glycol: Lactate interference
Limitations
- Lab error in measured value
- Late presentation: metabolites clear gap
- Hyperproteinemia/hyperlipidemia falsely lowers Na
Integration with Tools
- Corrected Sodium for glucose
- Anion Gap for acidosis
- Symptom Checker for AMS
FAQs
What if gap is negative? Lab variation or hyperlipidemia.
Can I use in children? Yes, same formula.
When to order methanol level? Gap >20 + acidosis.
Conclusion
The osmolar gap is a rapid screen for life-threatening ingestions. Use our 2025 Serum Osmolality Calculator in every unexplained coma or acidosis. Early detection of methanol or ethylene glycol saves vision, kidneys, and life. Always confirm with specific assays and treat empirically if delay.
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