Question 4.3

Define the terms 'base excess' and 'standard base excess' (20% marks)

[Click here to toggle visibility of the answers]

College Answer

Base excess is defined as the amount of acid or alkali that must be added to fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg.

Standard base excess is the amount of acid or alkali to return the ECF pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg and is calculated for blood at a Hb of 50 g/L.


In brief:

Base excess definition

  • Dose of acid or base required to return the pH of a blood sample
  • Measured at standard conditions: 37°C and 40mmHg PaCO2
  • Thus, isolates the metabolic disturbance from the respiratory

Standard base excess

  • Dose of acid or base required to return the pH of an anaemic blood sample
  • Calculated for a Hb of 50g/L
  • Haemoglobin buffers both the intravascular and the extravascular fluid
  • Thus, SBE assesses the buffering of the whole extracellular fluid, not just the haemoglobin-rich intravascular fluid


SIGGAARD‐ANDERSEN, O., and N. FOGH‐ANDERSEN. "Base excess or buffer base (strong ion difference) as measure of a non‐respiratory acid‐base disturbance." Acta Anaesthesiologica Scandinavica 39.s107 (1995): 123-128.

Ole Siggard-Andersen has his own website, which is an excellent anthology of acid-base information. This man has pioneered the concept of base excess in 1958, two years after his graduation from University of Copenhagen as candidatus medicinae (laudabilis præ ceteris et quidem egregie).

Kraut, Jeffrey A., and Nicolaos E. Madias. "Serum anion gap: its uses and limitations in clinical medicine." Clinical Journal of the American Society of Nephrology 2.1 (2007): 162-174.

Shock, Nathan W., and A. Baird Hastings. "Studies of the acid-base balance of the blood IV. Characterization and interpretation of displacement of the acid-base balance." Journal of Biological Chemistry 112.1 (1935): 239-262.