ObjectivesCO2content can be determined from the PCO2in an acidified (forces all CO2into solution) and diluted blood sample. However, PCO2concentrations measured in conventional blood gas analyzers are only correct for samples with a significant buffer capacity (such as whole blood), so that mixing with the PCO2in the rinse solution and tubing walls does not significantly change the sample PCO2. This study describes a calibration method and validation data for the Radiometer Medical ABL2 CO2electrode system to accurately measure unbuffered blood samples used in the determination of blood CO2content (or other aqueous fluids).DesignProspective, criterion standard.SettingLaboratory.Measurements and Main ResultsBlood samples (0.4 mL) were acidified and diluted with 0.2 M lactic acid. After measuring PCO2, CO2content was calculated using the CO2solubility coefficient and the dilution factor of 20. CO2content was determined in a series of sodium carbonate (Na2CO2) solutions spanning the physiologic range of CO2content. Regression of the measured vs. the actual CO2content data generated a straight line with a slope of 0.796 and y-intercept of 12.5 (r2equals .99; n equals 48). These coefficients were successfully used to correct CO2content determined in blood samples into which graduated amounts of sodium carbonate were added.ConclusionsThis calibration procedure allows accurate measurement of PCO sub 2 in aqueous samples using the Radiometer ABL2 electrode system, and should be applicable to other blood gas analyzers. Necessary syringes and chemicals are readily available, the method is fast and simple, and the sample volume is small. In the practice of critical care medicine, accurate PCO2measurement in aqueous acidified and diluted blood provides direct determination of blood CO2content (useful in calculations of modified Fick cardiac output or tissue CO2production). Determinations of absolute CO2content in blood requiring complex methodology are not necessary. In addition, accurate measurement of aqueous gastric PCO2can help determine gastric pH, which is an important marker of tissue perfusion.(Crit Care Med 1996; 24:1215-1218)