Diagnosing metabolic acidosis in the critically ill: bridging the anion gap, Stewart, and base excess methods

Metabolic acid-base disorders are common in critically ill patients. Clinicians may have difficulty recognizing their presence when multiple metabolic acid-base derangements are present in a single patient. Clinicians should be able to identify the components of complex metabolic acid-base disorders since metabolic acidoses due to unmeasured anions are associated with increased mortality in critically ill patients. This review presents the derivation of three commonly used methods of acid-base analysis, which include the anion gap, Stewart physiochemical, and modified base excess. Clinical examples are also provided to demonstrate the subtleties of the different methods and to demonstrate their application to real patient data.

A comparison of these methods shows that each one is equally adept at identifying a metabolic acidosis due to unmeasured anions; however, the Stewart physiochemical and the modified base excess methods better evaluate complex metabolic acid-base disorders.

While all three methods correctly identify metabolic acidosis due to unmeasured anions, which is a predictor of mortality, it remains unclear if further delineation of complex metabolic acid-base disorders using the Stewart physiochemical or the modified base excess methods is clinically beneficial.