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

Abstract

Background 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. Method 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. Results 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. Conclusions 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.