anion gap

Last reviewed 01/2018

The anion gap is a method of assessing the contribution of unmeasured anions to acidosis. It is calculated as a difference between the total of sodium and potassium ion concentration, minus the total of chloride and bicarbonate concentration. Some people omit the potassium. Thus:

  • Anion Gap = [K+] + [Na+] - [Cl-] - [HCO3-] (plasma concentrations)

The normal range for the anion gap is 6 - 16 mmol/l (1). The anion gap provides a measure of the difference between unestimated anions - phosphate, acetate and ketones - and cations.

The anion gap is likely to abnormally high in most conditions of acidosis except:

  • renal tubular acidosis
  • treatment with acetazolamide
  • ureteric implantation into colon

Note that reference ranges may vary between laboratories.


  1. Joint Pathology Services Royal Liverpool & Broadgreen University Hospitals (September 2006)