Emergency Medicine Emergency Medicine · High Yield

Metabolic Acidosis — MUDPILES

Metabolic acidosis with a raised anion gap has a defined differential diagnosis captured by the MUDPILES mnemonic. The anion gap directs investigation.

✦ The Mnemonic

"MUDPILES — Raised Anion Gap Metabolic Acidosis"

Anion gap = Na+ − (Cl− + HCO3−); normal 8–16 mEq/L; raised = unmeasured anions

M Methanol Toxic alcohol; formic acid → visual disturbance, blindness; treat with fomepizole

U Uraemia Renal failure — accumulation of organic acids; treat underlying renal disease

D DKA Diabetic ketoacidosis — beta-hydroxybutyrate; most common HAGMA in clinical practice

P Propylene glycol / Paracetamol Paracetamol toxicity: 5-oxoproline accumulation (chronic overdose); propylene glycol in IV lorazepam

I Isoniazid / Iron Isoniazid — lactic acidosis from mitochondrial dysfunction; Iron toxicity

L Lactic acidosis (Type A + B) Type A: tissue hypoperfusion (shock, sepsis); Type B: metformin, liver failure, malignancy

E Ethylene glycol Antifreeze — oxalic acid → renal failure + calcium oxalate crystals in urine; treat with fomepizole

S Salicylates Aspirin overdose — mixed respiratory alkalosis + metabolic acidosis; urinary alkalinisation

📚 Clinical Breakdown

Anion gap calculation: AG = Na+ − (Cl− + HCO3−). Normal = 8–16 mEq/L. A raised AG means unmeasured anions are present. A normal AG with metabolic acidosis (hyperchloraemic) suggests GI (diarrhoea) or renal causes (RTA, Addison's).

Delta ratio (delta-delta): used to unmask a concurrent metabolic alkalosis or normal AG acidosis hiding behind a HAGMA. Delta ratio = (AG − 12) / (24 − HCO3). <0.4: pure normal AG acidosis; 0.4–0.8: mixed HAGMA + normal AG; 1–2: pure HAGMA; >2: HAGMA + metabolic alkalosis.

Lactic acidosis: the most common cause is Type A (tissue hypoperfusion — sepsis, cardiogenic shock, limb ischaemia). Lactate >2 mmol/L = elevated; >4 mmol/L = severe. Treat the underlying cause. Metformin causes Type B lactic acidosis (rare) — hold in AKI, heart failure, and before IV contrast.

Normal anion gap 8–16 mEq/L

Most common HAGMA DKA (diabetic ketoacidosis)

Toxic alcohols antidote Fomepizole (alcohol dehydrogenase inhibitor)

Salicylate treatment IV sodium bicarbonate — urinary alkalinisation

⭐ Clinical Pearl

Osmolar gap: measured osmolality − calculated osmolality. Calculated = 2×Na + glucose + urea. Normal <10 mOsm/kg. A raised osmolar gap with HAGMA = toxic alcohol (methanol, ethylene glycol, isopropanol) until proven otherwise — treat with fomepizole and consider haemodialysis.

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