The Accuracy of Calculated Base Excess in Blood

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 . [{cHCO₃-(P)–c_7.4 HCO₃-(P)} + β. (pH–7.4)]. In simple approximation, where Z is a constant which depends only on total hemoglobin concentration (cHb, 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 αCO₂, mmol/l.mmHg) and in the apparent pK (pK'), but more significantly in the plasma bicarbonate concentration at reference pH (c>_7.4HCO₃-(P), mmol/l) and in Β, the slope of the CO₂-buffer line (mmol/l) for whole blood. Furthermore, the approximation was improved either by variation in Z (r_c), or in the apparent pK (pK') with changing pH. Thus, from a total of seven equations and from a reference set for pH, pCO₂ 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 pCO₂(12 to 96 mmHg), mean accuracy ΔBE, 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 (pK').

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 . pCO₂. 10^ph−6.1−24.26} + (9.5+1.63 . cHb) . (pH −7.4)] −0.2 . cHb . (1−sO₂), where the last term is a correction for oxygen saturation (sO₂). Hence, base excess can be obtained with high accuracy (<1 mmol/l) from the measured quantities of pH, pCO₂, cHb, and sO₂ in used.