Which substance is a classic unmeasured anion contributing to high anion gap metabolic acidosis?

• A. Methanol
• B. Chloride
• C. Sodium
• D. Bicarbonate

Answer: (A)

High anion gap metabolic acidosis happens when unmeasured anions build up in the blood. The measured ions we routinely look at are sodium, chloride, and bicarbonate, and the difference between the cations and the measured anions reveals the gap. A classic source of those unmeasured anions is formate from methanol metabolism. When methanol is ingested, it is turned into formic acid in the body, and that formate sits as an unmeasured anion, pushing the anion gap higher.

Chloride tends to drive a different pattern called hyperchloremic acidosis, where the normal anion gap stays roughly the same even as acidosis develops. Sodium is a positive ion, not an acid, and bicarbonate is a base that buffers acid; their roles don’t produce an increased anion gap the way an accumulating unmeasured organic acid does. So methanol fits the concept of an unmeasured anion boosting the gap most clearly.