Sodium minus chloride and bicarbonate, normally 8 to 16 mmol per litre
Expansion
Anion gap = sodium - (chloride + bicarbonate), normally 8 to 16 mmol/l.
It represents the unmeasured anions, chiefly albumin, phosphate and sulphate.
Raised anion gap acidosis (added acid) - MUDPILES:
- M - Methanol
- U - Uraemia
- D - Diabetic ketoacidosis
- P - Paraldehyde, propylene glycol
- I - Isoniazid, iron
- L - Lactic acidosis
- E - Ethylene glycol
- S - Salicylates
Normal anion gap (hyperchloraemic) acidosis (bicarbonate lost) - diarrhoea, renal tubular acidosis, acetazolamide, ureteric diversion, and large volumes of 0.9 per cent saline.
Always correct for albumin, since a hypoalbuminaemic critically ill patient can have a normal measured gap despite a significant acidosis.
The delta ratio (rise in anion gap divided by fall in bicarbonate) detects mixed disorders: below 1 suggests a coexisting normal gap acidosis, above 2 a coexisting metabolic alkalosis.