Strong ions and the analysis of acid-base disturbances (Stewart approach)
- Richard H Sterns, MD
Richard H Sterns, MD
- Editor-in-Chief — Nephrology
- Section Editor — Fluid and Electrolytes
- Professor of Medicine
- University of Rochester School of Medicine and Dentistry
Most internists use an approach to acid-base disturbances that was popularized by Relman and Schwartz in the 1960s [1,2]. This "traditional" approach accepts the Brønsted-Lowry definitions of acids and bases. The hydrogen ion concentration is expressed in blood as a function of the ratio between the PCO2 and the serum bicarbonate concentration as defined by the following equation:
[H+] = K x PCO2/[HCO3-]
where K is the dissociation constant for the reaction and PCO2 is equal to the partial pressure of carbon dioxide.
An analysis of metabolic acidosis using the traditional approach is presented in detail separately. (See "Approach to the adult with metabolic acidosis".)
By comparison, many surgeons, critical care specialists, and anesthesiologists have embraced an approach introduced by Peter Stewart in 1981 [3-7]. This alternative approach, termed "strong ion difference" (SID) or the Stewart approach, will be reviewed briefly in this topic.
- RELMAN AS. What are acids and bases? Am J Med 1954; 17:435.
- SCHWARTZ WB, RELMAN AS. A critique of the parameters used in the evaluation of acid-base disorders. "Whole-blood buffer base" and "standard bicarbonate" compared with blood pH and plasma bicarbonate concentration. N Engl J Med 1963; 268:1382.
- Stewart P. How to understand acid-base, Elsevier, New York 1981.
- Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol 1983; 61:1444.
- Fencl V, Leith DE. Stewart's quantitative acid-base chemistry: applications in biology and medicine. Respir Physiol 1993; 91:1.
- Kellum JA. Disorders of acid-base balance. Crit Care Med 2007; 35:2630.
- Rastegar A. Clinical utility of Stewart's method in diagnosis and management of acid-base disorders. Clin J Am Soc Nephrol 2009; 4:1267.
- Corey HE. Stewart and beyond: new models of acid-base balance. Kidney Int 2003; 64:777.
- Figge J, Mydosh T, Fencl V. Serum proteins and acid-base equilibria: a follow-up. J Lab Clin Med 1992; 120:713.
- Watson PD. Modeling the effects of proteins on pH in plasma. J Appl Physiol (1985) 1999; 86:1421.
- Carreira F, Anderson RJ. Assessing metabolic acidosis in the intensive care unit: does the method make a difference? Crit Care Med 2004; 32:1227.
- Balasubramanyan N, Havens PL, Hoffman GM. Unmeasured anions identified by the Fencl-Stewart method predict mortality better than base excess, anion gap, and lactate in patients in the pediatric intensive care unit. Crit Care Med 1999; 27:1577.
- Feldman M, Soni N, Dickson B. Influence of hypoalbuminemia or hyperalbuminemia on the serum anion gap. J Lab Clin Med 2005; 146:317.
- Nguyen BV, Vincent JL, Hamm JB, et al. The reproducibility of Stewart parameters for acid-base diagnosis using two central laboratory analyzers. Anesth Analg 2009; 109:1517.
- Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore) 1977; 56:38.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998; 26:1807.
- Corey HE. The anion gap (AG): studies in the nephrotic syndrome and diabetic ketoacidosis (DKA). J Lab Clin Med 2006; 147:121.
- Moviat M, van Haren F, van der Hoeven H. Conventional or physicochemical approach in intensive care unit patients with metabolic acidosis. Crit Care 2003; 7:R41.
- Dubin A, Menises MM, Masevicius FD, et al. Comparison of three different methods of evaluation of metabolic acid-base disorders. Crit Care Med 2007; 35:1264.
- Morimatsu H, Rocktäschel J, Bellomo R, et al. Comparison of point-of-care versus central laboratory measurement of electrolyte concentrations on calculations of the anion gap and the strong ion difference. Anesthesiology 2003; 98:1077.
- Cusack RJ, Rhodes A, Lochhead P, et al. The strong ion gap does not have prognostic value in critically ill patients in a mixed medical/surgical adult ICU. Intensive Care Med 2002; 28:864.
- Kaplan LJ, Kellum JA. Comparison of acid-base models for prediction of hospital mortality after trauma. Shock 2008; 29:662.
- Kurtz I, Kraut J, Ornekian V, Nguyen MK. Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches. Am J Physiol Renal Physiol 2008; 294:F1009.