The Science Journal of the American Association for Respiratory Care

Strong Ion Difference: Metabolic Acidosis or Alkalosis?

January 2002 / Volume 47 / Number 1 / Page 94

I hope you will be so kind as to enlighten me about a statement in an editorial published in RESPIRATORY CARE, “The Strong Ion Difference Approach: Can a Strong Case Be Made for Its Use in Acid-Base Analysis?” (Respir Care 1999;44(1):26-28). The following paragraph from that article is difficult to understand:

Likewise, the discerning reader will note a parallel similarity with SID [strong ion difference] and the classic anion gap, which are just offset by about 25 mEq/L, which is the sum of the HCO3 concentration and the difference between the other normally circulating strong ions (Ca++, Mg++, lactate, urate, sulfate) that are not included in the anion gap calculation. Therefore, an SID greater than 40 suggests a possible metabolic acidosis of either endogenous organic acid overproduction or under-clearance (lactic acid or ketoacid), intake of exogenous acid, or acid precursors (eg, methanol and salicylates). A nonrespiratory, normal SID acidosis suggests a hyperchloremic acidosis from loss of alkaline equivalents, and an SID lower than 40 is consistent with a metabolic alkalosis or a surfeit of unmeasured cations (as in cationic paraproteinemias or ingestion of cations, such as Mg++ and Ba++).1

I think that SID > 40 is associated with metabolic alkalosis and, conversely, SID < 40 is associated with metabolic acidosis, but the latter passage says just the opposite. SID is the same as buffer base described by Singer and Hastings in 1948.2 By definition, buffer base is increased in metabolic alkalosis and decreased in metabolic acidosis.

Aquiles J Roncoroni MD
University of Buenos Aires
Buenos Aires, Argentina


  1. Swenson E. The strong ion difference approach: can a strong case be made for its use in acid-base analysis? Respir Care 1999;44(1):26-28.
  2. Singer RB, Hastings AB. Improved clinical method for estimation of disturbances of acid-base balance of human blood. Medicine 1948;27:223-242.

The author responds:

Dr Roncoroni is entirely correct in catching my mistake. An SID greater than normal is evidence for a metabolic alkalosis, and vice versa. I thank him for pointing this out.

Erik R Swenson MD
Pulmonary and Critical Care Medicine
Veterans Affairs Puget Sound Health Care System
University of Washington
Seattle, Washington

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