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Non-Anion Gap Acidosis Diarrhoeal Bicarbonate Loss — ESENeph MCQ

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ModerateAcid-BaseNon-Anion Gap Acidosis Diarrhoeal Bicarbonate LossESENeph

A 28-year-old man presents with a pH of 7.32, PaCO2 2.8 kPa, bicarbonate 10 mmol/L, Na+ 140, K+ 3.0, Cl- 116 mmol/L, glucose 5.0, lactate 1.0 mmol/L. Anion gap = 14 mmol/L. He has chronic diarrhoea from Crohn disease. What acid-base disorder is present?

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Correct answer: D — Pure non-anion gap (hyperchloraemic) metabolic acidosis

Anion gap = 140 - (116 + 10) = 14 mmol/L. This is at the upper limit of normal (reference 8-12 without albumin correction, up to 14-16 with some references). The markedly elevated chloride (116 mmol/L) and very low bicarbonate (10 mmol/L) with near-normal anion gap indicate a predominantly non-anion gap (hyperchloraemic) metabolic acidosis. The cause is GI bicarbonate loss from chronic diarrhoea (Crohn disease) — the colon and ileum secrete bicarbonate in exchange for chloride, so diarrhoeal losses cause simultaneous bicarbonate depletion and chloride retention. Hypokalaemia (K+ 3.0) occurs because renal compensation for acidosis wastes potassium. The low PaCO2 (2.8 kPa) represents appropriate respiratory compensation (Winter formula check).

Reference: https://guidelines.ukkidney.org