The four things that cause an anion gap metabolic acidosis

Anion gap metabolic acidosis is one of those classic differentials that we all learn. My favourite approach is not the MUDPILES that I learnt in medical school, but rather a differential based on the four basic things that can cause an increase in unmeasured anions:

  1. Ketoacidosis: this includes your DKA, starvation ketoacidosis, etc.
  2. Lactic acidosis: often from poor perfusion, as in the various causes of shock.
  3. Renal failure: kidney failure alone can raise your anion gap, particularly from decreased clearance of NH4+.
  4. Toxins: ethanol, methanol, ethylene glycol, acetaminophen, and aspirin are some big ones here.

And, for interest’s sake: although the most common form of lactic acidosis is from L-lactate (that’s what the lab reports as “lactate”), there’s also the rare D-lactic acidosis. It’s caused by gut bacteria in patients with jejunoileal bypass, short bowel syndrome, and intestinal obstruction. The bacteria create D-lactate rather than our normal human L-lactate.

Read more: Metabolic Acidosis in Chapter 66: Acidosis and Alkalosis, Harrison’s Principles of Internal Medicine 19e.

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