Vomiting Does NOT Cause Anion Gap Acidosis
Vomiting typically causes a normal anion gap metabolic alkalosis, not acidosis, due to loss of hydrochloric acid from gastric secretions. 1
Metabolic Effects of Vomiting
The classic acid-base disturbance from vomiting is:
- Hypochloremic, hypokalemic metabolic alkalosis with a normal anion gap 1
- Loss of gastric HCl leads to increased serum bicarbonate 1
- Paradoxical aciduria may occur despite systemic alkalosis 1
- Dehydration and volume depletion compound the metabolic derangement 1
When Vomiting Appears with Anion Gap Acidosis
If a patient presents with both vomiting and anion gap acidosis, these are typically separate processes, not cause-and-effect:
The vomiting is a symptom of the underlying condition causing acidosis:
- Diabetic ketoacidosis - vomiting occurs as a symptom of the metabolic crisis 2, 3
- Uremia/renal failure - nausea and vomiting accompany accumulation of organic acids 2
- Toxic ingestions (methanol, ethylene glycol, salicylates) - GI symptoms including vomiting occur alongside the anion gap acidosis 2, 4
- Lactic acidosis from sepsis - vomiting may be present as part of the systemic illness 5
Rare Exception: D-Lactic Acidosis
- Bacterial overgrowth in severe gut dysmotility can produce D-lactic acid, causing high anion gap acidosis 6
- This occurs in the context of gut stasis with large volume vomiting, but the acidosis is from bacterial production of D-lactate, not from the vomiting itself 6
- The vomiting and acidosis are both consequences of the underlying dysmotility 6
Clinical Pitfall to Avoid
Do not assume vomiting explains an anion gap acidosis. If you encounter a patient with vomiting and find anion gap acidosis on labs, immediately search for the true cause using the standard differential 2, 3:
- Check glucose and ketones (diabetic ketoacidosis) 3, 7
- Measure lactate (lactic acidosis) 2, 7
- Assess renal function (uremic acidosis) 2, 7
- Calculate osmolar gap and check toxic alcohol levels if suspected 2, 7
- Consider salicylate level 2
The presence of vomiting should prompt consideration of metabolic alkalosis as a concurrent process, creating a mixed acid-base disorder 1.