Metabolic Acidosis

Key points

• Metabolic acidosis can be classified using the anion gap. The commonly used MRCP formula is AG = Na⁺ − (Cl⁻ + HCO₃⁻) ; potassium is usually omitted.
• Normal-anion-gap metabolic acidosis (NAGMA) includes bicarbonate loss or impaired renal acid excretion, particularly diarrhoea and renal tubular acidosis.
• Acetazolamide causes proximal bicarbonate wasting and can produce a normal-anion-gap metabolic acidosis .
• Ammonium chloride administration increases acid load and can produce a normal-anion-gap metabolic acidosis; it is a classic exam example rather than a routine clinical treatment.
• High-anion-gap metabolic acidosis has a broad differential including lactate, ketoacidosis, advanced renal failure, toxins and other unmeasured anions; do not classify Addison disease itself as simply 'raised anion gap'.
• Primary adrenal insufficiency (Addison disease) can cause hyperkalaemic normal-anion-gap metabolic acidosis through reduced aldosterone effect/type 4 RTA physiology.
• MRCP discriminator: HAGMA → lactate/ketoacidosis/renal failure/toxins; NAGMA → diarrhoea/RTA/acetazolamide.
• Potassium clue: Addison disease/type 4 RTA causes hyperkalaemic NAGMA, not a primary high-anion-gap acidosis.