Anaes · Applied cardiovascular & respiratory physiology
Acid-base: the Stewart approach
Also known as Stewart acid-base · Strong ion difference · SID · Total weak acid · Atot · Physicochemical acid-base
Peter Stewart's physicochemical approach reinterprets acid-base in terms of three independent variables that determine the dependent variable, pH, by mass-action: the carbon dioxide tension, the strong ion difference (the net charge of fully dissociated ions, dominated by sodium and chloride), and the total concentration of weak acid (chiefly albumin and phosphate). The framework rests on five exam-critical ideas: only three independent variables set the pH — PaCO2, the strong ion difference, and total weak acid; the strong ion difference (normally about 38 to 42 mmol per litre, dominated by sodium minus chloride) is the principal metabolic lever, and a fall in the SID (a chloride rise) causes metabolic acidosis while a rise causes alkalosis; total weak acid is mainly albumin, so hypoalbuminaemia causes a metabolic alkalosis that can mask a coexisting acidosis; bicarbonate is a DEPENDENT variable in Stewart's view (it changes to maintain electroneutrality), not a cause; and the Stewart approach explains the hyperchloraemic metabolic acidosis of large-volume normal saline, the hypoalbuminaemic alkalosis of critical illness, and unmeasured anions (the strong ion gap) that the bicarbonate method can miss. Built on the Stewart-framework reviews (Boyle 2002; Kellum 2005; Story 2004), the Figge serum-protein (1991) and anion-gap correction (1998) studies, the SMART (2018) and SALT-ED (2018) balanced-crystalloid trials, the paediatric DKA fluids trial (Sweety 2026), the dialysis Boston-Stewart comparison (Kroustalakis 2025), the neonatal fluids commentary (Carrigan 2026), and the robotic-surgery acid-base study (Pitimada 2026).
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Red flags
- In the Stewart approach only THREE variables independently set the pH: PaCO2, the strong ion difference (SID), and total weak acid (Atot). Bicarbonate is a dependent variable, not a cause.
- A fall in the strong ion difference — classically from a rise in chloride after large-volume 0.9 percent saline — causes a hyperchloraemic (normal-anion-gap) metabolic acidosis; balanced crystalloids reduce it.
- Hypoalbuminaemia lowers total weak acid and produces a metabolic alkalosis that can mask a coexisting metabolic acidosis — albumin must be corrected for when interpreting a base excess or anion gap.
- The strong ion gap (SIG) detects unmeasured anions (lactate, toxins) that the conventional anion gap can miss in the hypoalbuminaemic critically ill patient.
- Stewart and the bicarbonate/base-excess approaches agree on the four primary disorders; Stewart adds mechanistic clarity around chloride, albumin and unmeasured ions, where the bicarbonate method can be misleading.
Meet the patient
A postoperative patient has received five litres of 0.9 percent saline and now has a pH of 7.28 with a normal lactate. The traditional reading calls it a metabolic acidosis of unclear cause; Stewart calls it by its mechanism — chloride has risen, the strong ion difference has narrowed, and the equilibria have forced the hydrogen ion up. Knowing why changes the treatment: stop the saline, reach for a balanced crystalloid.[1]
Hold the one idea and the rest follows: bicarbonate is not a cause, it is a consequence. Once you accept that only PaCO2, SID and Atot move the pH, every saline acidosis, every phantom alkalosis and every false-normal anion gap resolves into mechanism.[1]
References11ShowHide
- [1]Boyle M, Baldwin I Introduction to an alternate view of acid/base balance: the strong ion difference or Stewart approach Aust Crit Care, 2002.PMID 11979985
- [2]Kellum JA Determinants of plasma acid-base balance Crit Care Clin, 2005.PMID 15781166
- [3]Story DA Bench-to-bedside review: a brief history of clinical acid-base Crit Care, 2004.PMID 15312207
- [4]Figge J, Rossing TH, Fencl V The role of serum proteins in acid-base equilibria J Lab Clin Med, 1991.PMID 2045713
- [5]Figge J, Jabor A, Kazda A, Fencl V Anion gap and hypoalbuminemia Crit Care Med, 1998.PMID 9824071
- [6]Semler MW, et al. Balanced Crystalloids versus Saline in Critically Ill Adults N Engl J Med, 2018.PMID 29485925
- [7]Self WH, et al. Balanced Crystalloids versus Saline in Noncritically Ill Adults N Engl J Med, 2018.PMID 29485926
- [8]Kroustalakis N, et al. Dialysis and Acid-Base Balance: A Comparative Physiological Analysis of Boston and Stewart Models J Clin Med, 2025.PMID 41303241
- [9]Carrigan K, Lakshminrusimha S Balanced fluid bolus: Should we prefer balanced crystalloids over normal saline? J Perinatol, 2026.PMID 41826663
- [10]Sweety S, Panda S, Das RR Balanced electrolyte solution vs isotonic saline in the resuscitation of children with diabetic ketoacidosis: A randomized controlled trial World J Clin Pediatr, 2026.PMID 41884024
- [11]Pitimada M, et al. Acid-base balance and respiratory mechanics during robotic-assisted surgery: an observational study J Robot Surg, 2026.PMID 42010144