Makindo Medical Notes

Related Subjects: |Metabolic acidosis |Anion Gap

Elevated anion gap should be regarded as reflecting a life-threatening abnormality until proven otherwise. It may reflect a high lactate, uraemia or methanol/ethylene glycol poisoning or ketoacidosis.

Definition and Normal Values

• AG = Na⁺ - Cl⁺ - HCO^-₃ normal value is < 12 mmol/l is the commonest mentioned formula
• AG = Na⁺ + K⁺ - [Cl^- + HCO^-₃] normal value is 10-18 mmol/l and is the formula used in MRCP

HAGMA Metabolic acidosis + Raised Anion gap > 12 (Normal chloride)

• Ketones: DKA, Alcoholic ketoacidosis, Starvation ketoacidosis
• Uraemic acidosis with renal failure
• Lactic acidosis: Carbon monoxide, Cyanide, Metformin, Ischemia, shock
• Paraldehyde, Isoniazid, Iron
• Methanol, Ethylene glycol, salicylates

NAGMA Metabolic acidosis + Normal Anion gap < 12 (Hyperchloraemic)

• Diarrhoea, small intestinal fistula, pancreatic alkali losses
• Carbonic anhydrase inhibitors e.g. acetazolamide
• Renal tubular acidosis, uretoenterostomy
• Rapid hydration with NaCl
• Addison's disease
• Ammonium chloride injection

References

• Serum Anion Gap Its Uses and Limitations in Clinical Medicine
• Anion-gap metabolic acidosis Internet Book of Critical care

• 🩸 Case 1 – Age 70 (Raised Anion Gap): Elderly diabetic man presented drowsy with Kussmaul respirations and fruity odour. ABG: pH 7.12, HCO₃⁻ 10 mmol/L, anion gap 26. Glucose 28 mmol/L, ketones +++. Diagnosis: Diabetic ketoacidosis (DKA). Management: IV insulin infusion, fluids, and potassium replacement. Teaching point: DKA causes a high anion gap metabolic acidosis due to accumulation of ketoacids (β-hydroxybutyrate and acetoacetate) — hallmark of uncontrolled diabetes.
• ☣️ Case 2 – Age 45 (Raised Anion Gap): Found confused with tachypnoea and hypotension after suspected paracetamol overdose. ABG: pH 7.19, HCO₃⁻ 9 mmol/L, anion gap 28. Lactate markedly elevated. Diagnosis: Lactic acidosis secondary to hepatic failure and hypoperfusion. Management: Fluid resuscitation, N-acetylcysteine, and ICU support. Teaching point: Lactic acidosis is the commonest high-anion-gap acidosis in hospital — look for tissue hypoxia, sepsis, or toxins impairing oxidative metabolism.
• 🚽 Case 3 – Age 60 (Normal Anion Gap): Man with long-standing diarrhoea from ulcerative colitis flare presented dehydrated and acidotic. ABG: pH 7.28, HCO₃⁻ 15 mmol/L, anion gap 10, normal lactate. Diagnosis: Non-anion-gap (hyperchloraemic) metabolic acidosis due to bicarbonate loss in stool. Management: IV rehydration, correction of electrolytes, and treatment of colitis. Teaching point: Normal-anion-gap acidosis occurs with direct loss of bicarbonate or renal tubular dysfunction — chloride rises to maintain electroneutrality.