Comparative Study of Values of Calculated Bicarbonate and Measured Total Carbon Dioxide Content

Abstract

Bicarbonate [HCO3-] can be calculated using the Henderson-Hasselbalch equation and direct measurements of pH and pCO2. In critically ill patients, studies show discrepancies between calculated [HCO3-] and measured total carbon dioxide content (TCO2) obtained with different analyzers. These discrepancies are caused by variations in the dissociation constant (pK1′) and by other confounding factors. The authors' goal was to determine whether discrepancies between calculated [HCO3-] and measured TCO2still exist when using modern whole-blood analyzers. They compared calculated [HCO3-] obtained with three critical care analyzers with measured TCO2from two routine chemistry analyzers. Arterial whole blood from critically ill patients and venous serum from hospitalized patients were studied. They obtained calculated [HCO3-] from the OPTI CCA, OMNI 9, and Stat Profile Ultra C. TCO2was obtained using the Dimension RxL and the CRT. The authors found that most calculated [HCO3-] and measured TCO2correlated well using least squares linear regression. However, poor agreement of calculated [HCO3-] versus measured TCO2was found using Bland-Altman analysis and Story and Poustie's criteria. In conclusion, discrepancies still occur in mean bias for calculated [HCO3-] versus measured TCO2and the bias may be clinically important when making decisions about the treatment of acid-base disorders. © 2003 Lippincott Williams & Wilkins, Inc.