The osmolal gap detects unmeasured osmotically active substances — most importantly toxic alcohols like methanol and ethylene glycol. Enter your values below for instant results.
Enter lab values to compare measured vs calculated osmolality and detect unmeasured osmoles.
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How to calculate and interpret the osmolal gap.
This estimates osmolality from the three main contributors: sodium (and its paired anions), glucose, and urea. Some formulas add ethanol/4.6 to account for alcohol consumption.
The difference between what the osmometer actually measures and what you can account for. Normal is -10 to +10 mOsm/kg. A gap above 10 means something unmeasured is present — often a toxic alcohol.
Updates in real-time as you change values.
Detecting invisible poisons in the blood.
The osmolal gap is the difference between the measured serum osmolality (from the lab) and the calculated osmolality (estimated from known solutes). Normally, these match closely. When they don't, something unmeasured is contributing to the osmolality.
The most dangerous cause of an elevated osmolal gap is toxic alcohol ingestion. Methanol (found in windshield fluid) and ethylene glycol (antifreeze) are odorless and can be fatal. Early in the course, the parent compound raises osmolality before it's metabolized into acids. The osmolal gap catches what the anion gap might miss in the first hours after ingestion.
What's normal and what's concerning.
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Serum Osmolality | 275 – 295 | mOsm/kg | Measured by freezing point depression |
| Normal Osmolal Gap | -10 to +10 | mOsm/kg | Difference between measured and calculated |
| Concerning Gap | > 10 | mOsm/kg | Suggests unmeasured osmoles |
| Highly Suspicious | > 25 | mOsm/kg | Strong suspicion for toxic alcohol |
Gauge updates as you change values.
What different osmolal gap values suggest.