How it's calculated
Calculated osmolality = 2 × sodium + glucose/18 + BUN/2.8. The osmolal gap is the measured osmolality minus this calculated value. Glucose and BUN can be toggled to mmol/L (BUN toggles to its equivalent "Urea" value, the unit most non-US labs report directly). Measured osmolality can be toggled between mOsm/kg and mmol/kg — these are used interchangeably for this test and represent the same number.
Interpretation
| Osmolal gap | Interpretation |
|---|---|
| ≤ 10 mOsm/kg | Normal |
| > 10 mOsm/kg | Elevated — consider unmeasured osmoles |
Clinical use
- An elevated osmolal gap raises suspicion for ingestion of toxic alcohols (methanol, ethylene glycol, isopropanol) or other unmeasured osmotically active substances.
- Should be interpreted alongside the anion gap — a combined elevated osmolal gap and elevated anion gap metabolic acidosis is a classic pattern for methanol or ethylene glycol poisoning.
- A normal osmolal gap does not fully exclude toxic alcohol ingestion, particularly later in the course once the parent alcohol has been metabolised to its toxic acid metabolites.
- Isopropanol ingestion characteristically raises the osmolal gap without a significant anion gap or metabolic acidosis, unlike methanol or ethylene glycol.
Frequently asked questions
What is the osmolal gap used for?
It screens for the presence of unmeasured osmotically active substances in the blood, most notably toxic alcohols such as methanol and ethylene glycol.
What is a normal osmolal gap?
Generally 10 mOsm/kg or less; higher values suggest an unmeasured substance is contributing to serum osmolality.
Can a normal osmolal gap rule out toxic alcohol poisoning?
Not completely — the gap can normalise later in the course once the parent alcohol has been metabolised, so a normal gap does not fully exclude ingestion, especially if presentation is delayed.
How does this relate to the anion gap?
The combination of an elevated osmolal gap with an elevated anion gap metabolic acidosis is a classic pattern seen in methanol or ethylene glycol poisoning.
References
- Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659.
- Krahn J, Khajuria A. Osmolality gaps: diagnostic accuracy and long-term variability. Clin Chem. 2006;52(4):737-739.