[Base Excess] vs [Strong ION Difference]

Which Is More Helpful?
  • Robert Schlichtig
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 411)


Blood [base excess] ([BE]) is defined as the change in [strong acid] or [strong base] needed to restore pH to normal at normal PCO2. Some believe that [BE] is unhelpful because [BE] may be elevated with a “normal” [strong ion difference] ([SID]), where a strong ion is one that is always dissociated in physiological solution, and where [SID] = [strong cations] — [strong anions]. Using a computer simulation, the hypothesis was tested that [SID] = [SID Excess] ([SIDEx]), where [SIDEx] is the change in [SID] needed to restore pH to normal at normal PCO2. The most current version of the plasma [SID] ([SID]p) equation was used as a template, and an [SIDEx] formula, of the Siggaard-Andersen form, derived: \({\left[ {{\text{SIDEx}}} \right]_p} = {\left[ {HC{O_3}^ - } \right]_P} - 24.72 + \left( {p{H_p} - 7.4} \right) \times \left( {1.159 \times {{\left[ {{\text{alb}}} \right]}_p} \times + 0.423 \times {{\left[ {Pi} \right]}_p}} \right)\). [SID] was compared to [SIDEx] over the physiologic range of plasma buffering, and it was found that [SIDEx] varied by ~ 15 mM at any given [SID], thereby faulting the hypothesis. It is concluded that [SID] can be “normal” with an elevated [SIDEx], the latter being an expression of the [BE] concept, and a more helpful quantity in physiology.

The “metabolic” componenet of a given acid-base disturbances is usually estimated as whole blood [base excess] ([BE]WB), where [BE]WB is defined as the change in [strong acid] or [strong base] need to restore plasma (pHp) to 7.4 at PCO2 of 40 Torr1–3. However. the [BE] approach has been criticized as “Inadequate for interpretation of complex acid-base derangements such as those seen in critically ill patient4–5.” The proposed alternative is the strong ion difference (SID) method, where a strong ion is one that is always dissociated in solution, and where [SID] = [strong cations - [strong anions]4–8.

On the one hand, it does not seem possible, by the definitions of these entities, to change [SID] without also changing [BE]. On the other hand, a selected group of critically ill patients with hypoproteinemia has been reported in whom [SID] was “normal” (i.e. ∼ 40mEq11) but [BE]WB clearly increased4,5,9. The idea was that hypoproteinemia caused the alkalosis, due to a deficiency of plasma weak acid buffer, necessitating increased [HCO3]p to maintain electrical neutrality. How could [SID] be “normal”, but [BE] increased? The purpose of the current exercise was to address this question. An [SID excess] ([SIDex]) formula was developed, conceptually identical to Siggaard-Andersen’s [BE], and [SID] was compared to [SIDex] over the physiological range of plasma [albumin] ([alb]p), plasma [phosphate] ([Pi]p), and plasma pH (pHp).


Base Excess Strong Anion Current Exercise Normal PCO2 Powerful Clinical Tool 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    Siggaard-Andersen, O. The acid-base status of blood (Fourth Edition), Copenhagen, Munksgaard, 1974, pp. 80–83.Google Scholar
  2. 2.
    Siggaard-Andersen, O. The Van Slyke equation. Scand. J. Clin. Lab. Invest. 37; 15–20, Suppl. 146, 1977.CrossRefGoogle Scholar
  3. 3.
    Siggaard-Andersen, O., Wimberley PD, Fogh-Andersen N, Gøthgen IH. Measured and derived quantities with modern pH and blood gas equipment: calculation algorithms with 54 equations. Scand. J. Clin. Lab. Invest. 48, Supp. 198, 7–15, 1988.Google Scholar
  4. 4.
    Fencl V, Leith DE. Stewart’s quantitative acid-base chemistry: applications in biology and medicine. Resp. Physiol. 91: 1–16, 1994.CrossRefGoogle Scholar
  5. 5.
    Fencl V, Rossing TH. Acid-base disorders in critical care medicine. Ann. Rev. Med. 40: 17–29, 1989.PubMedCrossRefGoogle Scholar
  6. 6.
    Stewart, P.A. How to Understand Acid-Base: A Quantitative Acid-Base Primer for Biology and Medicine. New York: Elsevier/North-Holland, 1981, Table 7.3.Google Scholar
  7. 7.
    Stewart, P.A. Modern quantitative acid-base chemistry. Can. J. Physiol. Pharmacol. 61: 1444–1461, 1983.PubMedCrossRefGoogle Scholar
  8. 8.
    Figge, J., T. Mydosh, and V. Fencl. Serum proteins and acid-base equilibria: a follow-up. J. Lab. Clin. Med. 120:713–719, 1992.PubMedGoogle Scholar
  9. 9.
    McAuliffe JJ, Lind LJ, Leith DE, Fencl V. Hyproproteinemic alkalosis. Am. J. Med. 81: 86–90, 1986.PubMedCrossRefGoogle Scholar
  10. 10.
    Rossing TH, Maffeo N, Fencl V. Acid-base effects of altering plasma protein concentration in human blood in vitro. J. Appl. Physiol. 61: 2260–2265, 1986PubMedGoogle Scholar
  11. 11.
    Courtney ME, Greene HL, Folk CC, Helinek GL, Dmitruk A. Rapidly declining serum albumin values in newly hospitalized patients: prevalence, severity, and contributory factors. JPEN 6: 143–145, 1982.CrossRefGoogle Scholar
  12. 12.
    Sganga G, Siegel JH, Brown G, Coleman B, Wiles CE, Beizberg H, Wedel S. Reprioritization of hepatic plasma protein release in trauma and sepsis. Arch Surg 120: 187–199, 1985.PubMedCrossRefGoogle Scholar
  13. 13.
    Schlichtig R. Base excess — a powerful clinical tool in the ICU. Critical Care State of the Art, Society of Critical Care Medicine, volume 17, 1996 (in press).Google Scholar

Copyright information

© Springer Science+Business Media New York 1997

Authors and Affiliations

  • Robert Schlichtig
    • 1
  1. 1.Departments of Anesthesiology and Critical Care Medicine; Medicine; and SurgeryUniversity of Pittsburgh V.A. Medical CenterPittsburghUSA

Personalised recommendations