Acid Base Disorders • LITFL • CCC Investigations

Metabolic acidosis (reduced HCO3)

The first step involves calculating the anion gap, where there are different categories of differentials for either a normal anion gap metabolic acidosis (NAGMA) or a high anion gap metabolic acidosis (HAGMA)

Anion Gap = [Na+] – ([Cl–] + [HCO3–])

Normal anion gap metabolic acidosis (NAGMA)

A normal anion gap is 12 ± 4. Differentials for a normal anion gap metabolic acidosis can be remembered with “USED CRAP”

USED CRAP

• Ureterostomy
• Small bowel fistula
• Extra chloride (hyperchloraemic metabolic acidosis)
• Diarrhoea
• Carbonic anhydrase inhibitor
• Renal tubular acidosis
• Addison disease
• Pancreatic duodenal fistula

The anion gap can also be low (<3) or negative, with the differentials for a low or negative anion gap metabolic acidosis including:

• Analytical error (severe hypernatraemia, pseudohyponatraemia, hyperviscosity, hyperlipidaemia)
• Reduced unmeasured anions (dilution, hypoalbuminaemia)
• Increased unmeasured cations (lithium, calcium, magnesium, potassium)
• Pseudohyperchloraemia (bromide, iodide, salicylates, thiocyanate)

High anion gap metabolic acidosis (HAGMA)

The differentials for a high anion gap (>16) metabolic acidosis can be remembered with “CAT MUD PILES” or more simply “Left Total Knee Replacement (L TKR)”

CAT MUD PILES

• Carbon monoxide, cyanide
• Alcoholic ketoacidosis
• Toluene
• Methanol, metformin (phenformin)
• Uraemia
• Diabetic ketoacidosis, D-lactic acidosis
• Paracetamol, pyroglutamic acid, paraldehyde, propylene glycol
• Isoniazid, iron
• Lactate
• Ethanol, ethylene glycol
• Salicylates

L TKR

• Lactate
• Toxins
• Ketones
• Renal