The anion gap (AG) = Na⁺ − (Cl⁻ + HCO₃⁻). Normal range is approximately 8–12 mEq/L (some labs use 6–14). It represents unmeasured anions, mostly albumin, sulphate, phosphate, and organic acids. An elevated AG is the single most useful clue to organic acidosis.

Because albumin accounts for ~75% of the normal AG, the gap MUST be corrected for hypoalbuminaemia, which is rampant in Indian ICU patients (malnutrition, sepsis, CKD). The Figge correction:

Corrected AG = measured AG + 2.5 × (4 − serum albumin g/dL)

MUDPILES (classic): Methanol, Uraemia, DKA / Alcoholic ketoacidosis, Paraldehyde / Propylene glycol, Iron / Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.

GOLDMARK (modern, Lancet 2008): Glycols (ethylene, propylene), Oxoproline (chronic paracetamol), L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.

In Indian ED practice — DKA, septic lactic acidosis, AKI / CKD uraemia, methanol (hooch tragedies), salicylate (Disprin overdose), and metformin-associated lactic acidosis dominate.

The Δ/Δ ratio = (corrected AG − 12) / (24 − HCO₃). In a pure HAGMA, every mEq increase in AG should be matched by a 1 mEq fall in HCO₃ → ratio ≈ 1.

• <0.4 — combined HAGMA + NAGMA (e.g., DKA with diarrhoea)
• 0.4–1.0 — mixed HAGMA + NAGMA
• 1.0–2.0 — pure HAGMA
• >2.0 — HAGMA + metabolic alkalosis OR chronic respiratory acidosis

Ureterosigmoidostomy, Saline (large volume), Endocrine (Addison's, hyperparathyroidism), Diarrhoea, Carbonic anhydrase inhibitors (acetazolamide), Ammonium chloride / Aldosterone deficiency, Renal tubular acidosis, Pancreatic fistula.

The urine anion gap (UAG = Na + K − Cl) separates the two big buckets — negative UAG = GI bicarbonate loss (diarrhoea), positive UAG = renal acid-handling defect (RTA).