Tuesday, 26 October 2010

MRCP revision battle 35.6: Anion gap

If you have a patient with metabolic acidosis you need to calculate their anion gap.  This is very simple to do, so long as you can remember the following formula (which unfortunately I never can):

Anion gap = (Na + K) - (Cl + HCO3)

A 'normal' anion gap is 10 to 18mmol/l (which can be recalled as roughly the ages you attend secondary school for)

Causes of a raised anion gap may be recalled as LUKES:
  • lactic acid (shock, infection, hypoxia)
  • urate (renal failure)
  • ketones (diabetes, alcohol)
  • ethylene glycol/methanol
  • salicylate

A more comprehensive list can be recalled by 'cute dimples' = cyanide, urate, toulene, ethylene glycol, diabetic ketoacidosis, isoniazid, methanol, propylene glycol, lactic acid, salicylates

Causes of a metabolic acidosis with a normal anion gap can be recalled as FUSEDCARS:
  • fistula (pancreatic)
  • uretogastric conduits
  • saline administration
  • endocrine (hyperparathyroidism)
  • diarrhoea
  • carbonic anhydrase inhibitors (acetazolamide)
  • ammonium chloride
  • renal tubular acidosis
  • spironolactone

Hopefully thats demystified the anion gap for you.