## Abstract

Most equations used for calculation of the base excess (*BE*, mmol/l) in human blood are based on the fundamental equation derived by Siggaard-Andersen and called the Van Slyke equation: *BE* = Z . [{*c*HCO_{3}-(P)–*c*
_{7.4} HCO_{3}-(P)} + β. (pH–7.4)]. In simple approximation, where Z is a constant which depends only on total hemoglobin concentration (*c*Hb, g/dl) in blood, three equations were tested: the ones proposed by Siggaard-Andersen (SA), the National Committee for Clinical Laboratory Standards (NCCLS) or Zander (ZA). They differ only slightly in the solubility factor for carbon dioxide αCO_{2}, mmol/l.mmHg) and in the apparent p*K* (p*K*'), but more significantly in the plasma bicarbonate concentration at reference pH (*c>*
_{7.4}HCO_{3}-(P), mmol/l) and in Β, the slope of the CO_{2}-buffer line (mmol/l) for whole blood. Furthermore, the approximation was improved either by variation in Z (*r*
_{c}), or in the apparent p*K* (p*K*') with changing pH. Thus, from a total of seven equations and from a reference set for pH, *p*CO_{2} and *BE* taken from the literature (n=148), the base excess was calculated. Over the whole range of base excess (−30 to +30 mmol/l) and *p*CO_{2}(12 to 96 mmHg), mean accuracy Δ*BE*, mmol/l) was greatest in the simple equation according to Zander and decreased in the following order: ±0.86 (ZA); ±0.94 (ZA, *r*
_{c}); ±0.96 (SA, *r*
_{c}); ±1.03 (NCCLS, *r*
_{c}); ±1.40 (NCCLS); ±1.48 (SA); and ±1.50 (p*K'*).

For all clinical purposes, the Van Slyke equation according to Zander is the best choice and can be recommended in the following form: *BE* = (1−0.0143 . cHb) . [{0.0304 . *p*CO_{2}. 10^{ph−6.1}−24.26} + (9.5+1.63 . *c*Hb) . (pH −7.4)] −0.2 . cHb . (1−sO_{2}), where the last term is a correction for oxygen saturation (*s*O_{2}). Hence, base excess can be obtained with high accuracy (<1 mmol/l) from the measured quantities of pH, *p*CO_{2}, *c*Hb, and *s*O_{2} in used.

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