by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

Stewart's Strong Ion Difference (SID)

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In 1981 Peter A. Stewart published his book How to understand acid-base - A quantitative acid-base primer for biology and medicine.. Two year later, in 1983, he published a paper also describing his concept of employing Strong Ion Difference as an alternative means of assessing clinical acid-base disturbances.

Now, some twenty eight years later, Stewart's Textbook of Acid-Base edited by John Kellum and Paul Elbers is available via and via Lulu Marketplace.

Stewart's approach has been well reviewed by Morgan. In Addition Rastegar published a valuable summary indicating that SID is only of benefit infrequently. Below is a review of the essentials of SID as well as an outline of the principal sources of criticism.

More Controversy:

Stewart's proposal provided one more source of acid-base controversy. The underlying science and rationale were less a source of criticism than were:

Traditional Approach:

When we first study acid-base balance, it is too easy believe that the concentrations of the hydrogen and bicarbonate ions, [H+] and [HCO3-], are at the heart of the problem - are dominant forces. We do, after all, discuss them, measure them, and treat them: whatever an acid or a base does must be due to the pH, i.e., the concentration of H+. In addition [HCO3-] must surely determine the metabolic state.

Such thinking is obviously incorrect: in alkaline solutions there are almost no hydrogen ions present; so, whatever causes the evil behavior of an alkaline solution, the only thing that cannot be responsible is the hydrogen ion. And, clnically, both respiratory and metabolic changes affect the [HCO3-]. So, what is responsible for [H+] and [HCO3-]? Far from being central, or controlling, factors they actually depend on the concentrations of the other ions in solution. Although this should be apparent, it too often isn't. Stewart's method does re-emphasize these relationships.

[H+] [OH-]
[HCO3-] [CO32-]
[HA]  [A-]

Stewart's Dependent Variables:

Stewart listed a total of six ion concentrations as dependent: [H+], [OH-], [HCO3-], [CO3--2], [HA], [A-] (weak acids and ions). In-vivo and clinically, therefore, these are not subject to independent alteration. Their concentrations are governed by concentrations of other ions and molecules.


Stewart's Independent Variables:

There are three variables which are amenable to change in-vivo: partial pressure of carbon dioxide (PCO2), total weak non-volatile acids [ATOT], and net Strong Ion Difference [SID]. The influence of these three variables can be predicted through six simultaneous equations:

  1. [H+] x [OH-] = K 'w (water dissociation equilibrium)
  2. [H+] x [A-] = KA x [HA] (weak acid)
  3. [HA] + [A-] = [ATOT] (conservation of mass for "A")
  4. [H+] x [HCO3-] = KC x PCO2 (bicarbonate ion formation equilibrium)
  5. [H+] x [CO32-] = K3 x [HCO3-] (carbonate ion formation equilibrium)
  6. [SID] + [H+] - [HCO3-] - [A-] -[CO32-] - [OH-] = 0 (electrical neutrality)


Stewart showed that using the above equations, the concentration of each of the dependent variables was uniquely and independently determined by the three independent variables: PCO2, [ATOT], [SID]. The equations he derived were complex and involved 4th order polynomials. Not surprisingly, Stewart used a computer to derive the effects of the three Independent Variables:

Clinical Considerations:

Changes in acid-base status are either respiratory or non-repiratory, i.e., metabolic:

Changing [SID]:

[SID] can be changed by two principal methods:

Changing [ATOT]:

The non-volatile weak acids comprise inorganic phosphate, albumin and other plasma proteins. Making the greatest contribution to acid-base balance are the proteins, particularly albumin, which behave collectively as a weak acid. Hypoproteinemia, therefore, causes a base excess and vice versa.

Phosphate levels are normally so low that a significant fall is impossible. However, in renal failure, high phosphate levels contribute to the acidemia.

Pros and Cons:


For many acid-base disturbances the traditional approach to acid-base balance serves well. For the clinician, the three variables of greatest use are the pH, PCO2, and standard base excess (SBE). When is Stewart's approach justifed?

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Alan W. Grogono
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