What is the most common cause of metabolic acidosis with a normal anion gap, and which laboratory parameter helps identify it?

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Metabolic acidosis with a normal anion gap occurs when bicarbonate is lost and chloride rises to maintain electroneutrality. The most common cause is bicarbonate loss from the GI tract, such as diarrhea. The lab clue is a low bicarbonate with elevated chloride in the setting of a normal anion gap, i.e., hyperchloremic metabolic acidosis. The normal anion gap persists because the chloride increase offsets the bicarbonate loss in Na − (Cl + HCO3). By contrast, lactic acidosis would raise the anion gap, and respiratory acidosis is not a metabolic disorder.