Anion Gap

Key points

• Anion gap: the commonly used MRCP formula is AG = Na⁺ − (Cl⁻ + HCO₃⁻) . A typical normal range is about 8–12 mmol/L without potassium, but laboratory reference ranges vary.
• Normal-anion-gap metabolic acidosis includes diarrhoea, renal tubular acidosis and other bicarbonate-loss/acid-excretion disorders.
• Acetazolamide causes proximal bicarbonate wasting and therefore can produce normal-anion-gap metabolic acidosis.
• Ammonium chloride is an acid load that can produce normal-anion-gap metabolic acidosis and is a classic exam example.
• MRCP discriminator: low HCO₃⁻ + normal AG → think diarrhoea/RTA/acetazolamide; high AG → lactate, ketones, renal failure or toxins.
• Always interpret the anion gap with albumin and the clinical context; hypoalbuminaemia lowers the expected normal AG.