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Clin Chem Lab Med. 2008;46(1):15-7.

Comparative study of calculated and measured total carbon dioxide.

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  • 1Division of Clinical Chemistry, Department of Pathology, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand.



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.


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.


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).


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.

[PubMed - indexed for MEDLINE]
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