Anion Gap.

Anion Gap.

Gap = Na+ + K+ - Cl- - HCO3-

Some causes of metabolic acidosis, e.g., lactic acidosis, release anions into the extracellular fluid which are not normally measured. When this occurs there will be an unexpected discrepancy between the sums of the principal cations and anions. The usual sum is:

Gap   =   Na⁺   +   K⁺   -   Cl^-   -   HCO₃^-

  15    =   140   +   5     -  105  -     25     mMol/L

In addition to Cl^- + HCO₃^- there are extra unmeasured anions, e.g., lactate, phosphate, sulphate, which increase the "gap". A gap greater than 30 indicates a significant concentration of unmeasured anions. 

Because this calculation relies upon the accuracy of the other measurements, small errors in these large numbers cause a disproportionately great error in the "gap". If information is required about the unmeasured anions, it is probably more appropriate to measure their concentration, i.e., lactate in tissue hypoxia, 3-hydroxybutyrate in diabetic ketosis, and phosphate or sulfate in renal failure