Strong ion difference and strong anion gap: The Stewart approach to acid base disturbances

More than 20 years ago Stewart proposed a method for acid-base analysis moving from a new definition of neutrality, acidity and alkalinity. 10 years later Figge defined more precisely the quantitative role of plasmatic nonbicarbonate buffers. Scientific literature studying this physicochemical approach to acid-base balance found controversial results. The aim of this article is to review the basic principles of the approach to acid-base developed by Stewart and Figge. The definitions of apparent and effective strong ion difference (SID) and of strong ion gap (SIG) will be also reviewed considering the findings of the scientific literature regarding these two main parameters related to this approach. Several clinical conditions of acid-base imbalances will be reviewed using the physicochemical approach described by Stewart. He identified the independent variables determining pH; they include SID, carbon dioxide partial pressure and the serum concentration of weak acids. SIG reflects the contribution of unidentified anions to SID. The physicochemical approach to acid-base developed by Stewart and Figge provides more details about complex acid-base abnormalities than does the classic approaches developed by Henderson-Hasselbach and Siggaard-Andersen. However the prognostic role of SID and SIG is still not well established. Moreover, the Stewart and Figge approach requires complicated calculations at the bedside and a simpler adjustment of the anion gap for albumin concentration may result in the same clinical conclusions as the physicochemical approach. Even if the pure Stewart approach to acid-base is difficult to apply in daily clinical practice and its simplifications seem more practical, it is noteworthy that Stewart improved significantly the approach to acid-base abnormalities by providing a better understanding of many acid-base imbalances.