Comparison of Plasma Total Carbon Dioxide and Standard Hydrogen Carbonate in a Hospital Laboratory Setting

Background: Studies comparing venous total carbon dioxide (tCO ₂) and standard hydrogen carbonate (HCO ₃ ^–(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 ₃ ^–(P,st) requisition from any department at Odense University Hospital between 11th May 2021 and 1st June 2021 had tCO ₂ and HCO ₃ ^–(P,st) analysed simultaneously. TCO ₂ was measured on Cobas® 8000, c702 module, while HCO ₃ ^–(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 ₂ and 22.5 (2.9) mmol/L for HCO ₃ ^–(P,st). TCO ₂ range was 10.1–42.3 mmol/L and 11.7–41.4 mmol/L for HCO ₃ ^–(P,st). Linear regression showed that tCO ₂ (mmol/L) = −2.90 + 1.15 × HCO ₃ ^–(P,st) (mmol/L) with R ² = 0.81. Bias (mean (SD) difference) between tCO ₂ and HCO ₃ ^–(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 tCO ₂ and HCO ₃ ^–(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 ₃ ^–(P,st) was considered the gold standard. Of the samples investigated, 46.1% had a mean difference between tCO ₂ and HCO ₃ ^–(P,st) of 0–1 mmol/L and 30.3% had 1.1–2.0 mmol/L. Conclusions: Our results indicate that venous tCO ₂ and venous HCO ₃ ^–(P,st) can be used interchangeably in a hospital setting for workup of acid–base disorders.