Comparative study of calculated and measured total carbon dioxide

BACKGROUND: Total carbon dioxide content (TCO2) can be calculated from measured values of pH and pCO2 according to a simplified and standardized form of the Henderson-Hasselbalch equation, or measured directly. METHODS: We assessed the agreement between calculated TCO2 and measured TCO2 using a total of 74 blood samples. Calculated TCO2 was obtained using blood gas analysis of pH and pCO2 in arterial whole blood on a Nova Stat Profile Critical Care Xpress analyzer. Measured TCO2 was determined using a Dimension RxL analyzer on arterial plasma, and was used as the comparative method. Deming regression analysis, correlation coefficients, bias (Bland-Altman method) and Student's t-test were used for statistical analysis. RESULTS: Deming regression analysis showed a high degree of correlation between calculated and measured TCO2 (r=0.97). The slope (0.96; 95% CI=0.90- 1.02) of the regression line was close to 1 with a positive intercept (2.86 mmol/L; 95% CI=1.44-4.27), and the standard error of the estimate was 0.20 mmol/L. The mean bias was 1.94 mmol/L with a standard deviation of 1.69 mmol/L. The pCO2 values showed a significant effect on calculated TCO2. Most paired differences were within the 95% limits of agreement (-1.45 to 5.33 mmol/L). CONCLUSIONS: Calculated TCO2 determined using blood gas analysis agreed with measured TCO2 and may be used to assess acid-base imbalance. However, clinicians should be cautious if using this calculated value in the critically ill patient.