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

Studies comparing venous total carbon dioxide (tCO₂) and standard hydrogen carbonate (HCO₃^-(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.

All patients with an HCO₃^-(P,st) requisition from any department at Odense University Hospital between 11th May 2021 and 1st June 2021 had tCO₂ and HCO₃^-(P,st) analysed simultaneously. TCO₂ was measured on Cobas® 8000, c702 module, while HCO₃^-(P,st) was calculated based on measurements on ABL835 Flex.

From 1210 patients, mean (standard deviation (SD)) was 22.9 (3.7) mmol/L for tCO₂ and 22.5 (2.9) mmol/L for HCO₃^-(P,st). TCO₂ range was 10.1-42.3 mmol/L and 11.7-41.4 mmol/L for HCO₃^-(P,st). Linear regression showed that tCO₂ (mmol/L) = -2.90 + 1.15 × HCO₃^-(P,st) (mmol/L) with R² = 0.81. Bias (mean (SD) difference) between tCO₂ and HCO₃^-(P,st)) was 0.4 (1.7) mmol/L with a -5.0-9.6 mmol/L range. Limits of agreement was -2.90-3.70 mmol/L. Comparison of classification within, above or below reference interval for tCO₂ and HCO₃^-(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 HCO₃^-(P,st) was considered the gold standard. Of the samples investigated, 46.1% had a mean difference between tCO₂ and HCO₃^-(P,st) of 0-1 mmol/L and 30.3% had 1.1-2.0 mmol/L.

Our results indicate that venous tCO₂ and venous HCO₃^-(P,st) can be used interchangeably in a hospital setting for workup of acid-base disorders.