Anion-Gap Metabolic Acidosis
Definition
The metabolic acid-base disorder produced by accumulating formate in methanol poisoning. The anion gap (Na+ - [Cl- + HCO3-], normal 8-12 mEq/L) rises as formate replaces bicarbonate; values above 20 mEq/L are characteristic of significant methanol toxicity.
- Cause
- Formate accumulation from methanol metabolism
- Normal anion gap
- 8-12 mEq/L
- Significant toxicity marker
- Above 20 mEq/L
- Displaced buffer
- Bicarbonate
Common questions
Why does formate specifically drive the acidosis in methanol poisoning rather than methanol itself?+
Methanol itself is only mildly toxic; it is oxidised by alcohol dehydrogenase to formaldehyde and then to formate, and it is this formate that both damages the optic nerve and accumulates as an unmeasured anion, consuming bicarbonate and widening the gap.
How does this finding change acute treatment decisions?+
A markedly widened, formate-driven anion gap supports urgent antidote therapy such as fomepizole or ethanol to block further methanol metabolism, alongside bicarbonate correction and consideration of dialysis in severe cases, ahead of waiting for definitive methanol quantitation.
Related terms
- Alcohol Dehydrogenase (ADH)
- The hepatic enzyme (isoforms ADH1A, ADH1B, ADH1C) that catalyses the first oxidation step of both ethanol (to acetaldehyde) and methanol (to formaldehyde)...
- Congener Fingerprinting
- The use of the full volatile congener profile (fusel alcohols, acetaldehyde, ethyl acetate, pyridine, MIBK) in illicit liquor samples to establish batch...
- Fomepizole (4-Methylpyrazole)
- A specific competitive inhibitor of alcohol dehydrogenase (Ki approximately 0.1 µM), used as the preferred antidote for methanol poisoning in high-resource settings....
- Formate (Formic Acid)
- The terminal toxic metabolite of methanol oxidation (HCOO-, pKa 3.7). Formate inhibits cytochrome c oxidase (mitochondrial complex IV), causing optic nerve and...
- FSSAI Methanol Limit
- Under India's Food Safety and Standards (Alcoholic Beverages) Regulations 2018, the maximum methanol content permitted in legally produced and sold spirits is...
- Haemodialysis in Methanol Poisoning
- The most efficient treatment for removing both methanol and formate from circulation and correcting severe metabolic acidosis. The EXTRIP workgroup (US) and...
- Latency Period
- The 12-24 hour window between methanol ingestion and onset of severe symptoms (visual disturbance, metabolic acidosis), during which the patient may appear...
- Pyridine
- An aromatic nitrogen heterocycle used as a secondary denaturant in Indian denatured spirit formulations (at 1-2% v/v). Its presence in a headspace...
- Vitreous Humour
- The gel-filled interior of the eye, anatomically isolated from hepatic and gastric post-mortem redistribution sources. In forensic toxicology of hooch deaths, vitreous...