Comparison of plasma total carbon dioxide and standard hydrogen carbonate in a hospital laboratory setting

Abstract

Background: Studies comparing venous total carbon dioxide (tCO<sub>2</sub>) and standard hydrogen carbonate (HCO<sub>3</sub><sup>–</sup>(P,st)) has shown diverse results, and it is debatable whether these two parameters can be used interchangeably for workup of acid-base disorders in a hospital setting. 
 
 Method: All patients with an HCO<sub>3</sub><sup>–</sup>(P,st) requisition from any department at Odense University Hospital between 11<sup>th</sup> May 2021 and 1<sup>st</sup> June 2021 had tCO<sub>2</sub> and HCO<sub>3</sub><sup>–</sup>(P,st) analysed simultaneously. TCO<sub>2</sub> was measured on Cobas® 8000, c702 module, while HCO<sub>3</sub><sup>–</sup>(P,st) was calculated based on measurements on ABL835 Flex. 
 
 Results: From 1210 patients, mean (standard deviation (SD)) was 22.9 (3.7) mmol/L for tCO<sub>2</sub> and 22.5 (2.9) mmol/L for HCO<sub>3</sub><sup>–</sup>(P,st). TCO<sub>2</sub> range was 10.1-42.3 mmol/L and 11.7-41.4 mmol/L for HCO<sub>3</sub><sup>–</sup>(P,st). Linear regression showed that tCO<sub>2</sub> (mmol/L) = -2.90 + 1.15 x HCO<sub>3</sub><sup>–</sup>(P,st) (mmol/L) with R<sup>2</sup>=0.81. Bias (mean (SD) difference) between tCO<sub>2</sub> and HCO<sub>3</sub><sup>–</sup>(P,st) was 0.4 (1.7) mmol/L with a -5.0-9.6 mmol/L range. Limits of agreement was -2.90 to 3.70 mmol/L. Comparison of classification within, above or below reference interval for tCO<sub>2</sub> and HCO<sub>3</sub><sup>–</sup>(P,st) showed that 984 samples (81%) retained their classification. Only one sample (0.1%) would be severely misclassified (outside the respective reference intervals) if HCO<sub>3</sub><sup>–</sup>(P,st) was considered the gold standard. Of the samples investigated, 46.1% had a mean difference between tCO<sub>2</sub> and HCO<sub>3</sub><sup>–</sup>(P,st) of 0-1 mmol/L and 30.3% had 1.1-2.0 mmol/L.
 
 Conclusions: Our results indicate that venous tCO<sub>2</sub> and venous HCO<sub>3</sub><sup>–</sup>(P,st) can be used interchangeably in a hospital setting for workup of acid-base disorders.