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Comparative study of calculated and measured total carbon dioxide
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Anchalee Chittamma
Published/Copyright:
July 30, 2007
Abstract
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;46:15–7.
Keywords: acid-base imbalance; calculated total carbon dioxide (TCO2); carbon dioxide; critical care; measured TCO2
Received: 2006-12-31
Accepted: 2007-5-8
Published Online: 2007-07-30
Published in Print: 2008-01-01
©2008 by Walter de Gruyter Berlin New York
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Keywords for this article
acid-base imbalance;
calculated total carbon dioxide (TCO2);
carbon dioxide;
critical care;
measured TCO2
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