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:
- Ketoacidosis: this includes your DKA, starvation ketoacidosis, etc.
- Lactic acidosis: often from poor perfusion, as in the various causes of shock.
- Renal failure: kidney failure alone can raise your anion gap, particularly from decreased clearance of NH4+.
- 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.