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Anchalee Chittamma, Somlak Vanavanan

Comparative study of calculated and measured total carbon dioxide

Keywords: acid-base imbalance, calculated total carbon dioxide (TCO2), carbon dioxide, critical care, measured TCO2

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.

Clin Chem Lab Med 2008;46

Clinical Chemical Laboratory Medicine, Walter de Gruyter

Print ISSN: 1434-6621
Volume: 05/2007
Pages: - - -

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