What is the role of the anion gap (AG) calculation in the workup of metabolic acidosis?

Updated: Oct 10, 2018
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

The change in AG (delta AG) helps in detecting the presence of a second acid-base disorder in patients with an elevated AG. It is calculated by the following equation:

(AG - 10)/(24 - HCO3-)

A value less than 1 indicates that the drop in serum HCO3- is not accompanied by a corresponding increase in the AG. This suggests that a portion of the H+ load is not accompanied by an unmeasured anion and indicates the presence of a mixed metabolic acidosis (eg, a non-AG acidosis and a high-AG acidosis).

A value greater than 1.6 indicates that the drop in serum HCO3- is associated with a larger-than-expected increase in the AG. This would occur if the serum HCO3- level was higher than normal prior to the onset of the metabolic acidosis and then dropped below normal with the addition of H+ coupled to an unmeasured anion. This indicates the presence of a mixed metabolic acidosis and metabolic alkalosis.


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