1. The Anion Gap (AG)

The Anion Gap helps determine the cause of a metabolic acidosis. It represents the "unmeasured" anions in the plasma (like phosphates, sulfates, and proteins).

The Formula

$$ AG = ([Na^+] + [K^+]) - ([Cl^-] + [HCO_3^-]) $$

(Note: Some clinicians omit K+ as its concentration is low; always check which formula your local lab uses, but for MRCP, including K+ is standard).

Normal Range

$$ 10–14\ \text{mmol/L} (if K^+ is excluded) $$

$$ 14–18\ \text{mmol/L} (if K^+ is included) $$

High Anion Gap Metabolic Acidosis (HAGMA)

This occurs when "new" acids are added to the body. Use the mnemonic MUDPILES:

• M – Methanol
• U – Uraemia (Renal failure)
• D – Diabetic Ketoacidosis (DKA)
• P – Paracetamol / Propylene glycol
• I – Isoniazid / Iron
• L – Lactic Acidosis (Sepsis, shock)
• E – Ethylene glycol
• S – Salicylates (Aspirin)

2. The Delta Ratio