Question 1

What is the most common cause of an elevated osmolar gap?

Accepted Answer

Ethanol ingestion is the most common cause. In emergency departments, any patient with an elevated gap should have ethanol measured first. If ethanol is absent and the gap persists, toxic alcohol ingestion must be considered.

Question 2

Can a normal osmolar gap rule out toxic alcohol ingestion?

Accepted Answer

No. The gap may be normal if the alcohol was ingested hours earlier and has been metabolised. In these cases, the anion gap will be elevated (formic acid from methanol, glycolic/oxalic acid from ethylene glycol). Both the osmolar and anion gaps must be assessed together.

Question 3

How long after ingestion does the osmolar gap remain elevated?

Accepted Answer

Methanol and ethylene glycol have half-lives of several hours. The osmolar gap is highest within 1–4 hours of ingestion. Beyond 6–8 hours, the gap may normalise while the anion gap rises. Late presentation may show only an elevated anion gap.

Question 4

What is the role of the osmolar gap in monitoring treatment?

Accepted Answer

Serial gaps can track the effectiveness of fomepizole or ethanol therapy (which blocks alcohol dehydrogenase) and haemodialysis (which clears both the parent alcohol and its toxic metabolite).