Total Carbon Dioxide Measured by the Vitros Enzymatic Method

Abstract

We evaluated the performance of an enzymatic method using dry chemistry for serum total carbon dioxide (tCO₂) determination using a Vitros 500 analyser. Imprecision results were acceptable and the linearity was verified for concentrations within a range of 5.5–39.2 mmol/l, i.e. y_measured = 0.93 x_calculated +1.32, r = 0.99. The Vitros tCO₂ method was unaffected by haemoglobin at all concentrations tested. Significant interference was caused by bilirubin at concentrations higher than 30 μmol/l; the addition of bilirubin lowered the apparent values for tCO₂ dose-dependently. Serum tCO₂ results were practically the same as those for plasma. The reference interval for venous tCO₂ concentrations in a healthy population was: 22.4–34.2 mmol/l (mean: 28.3 mmol/l). Comparison of venous serum tCO₂ results assayed using the Vitros method with bicarbonate (HCO₃⁻) values calculated by blood gas determination of pCO₂ and pH in arterial blood samples gave poor agreement, r = 0.58. The data revealed a mean difference of 5.48 ± 3.09 mmol/l between the tCO₂ measurements and calculated bicarbonate. This was statistically (p = 0.01) and clinically significant. We conclude that the Vitros method provides reliable tCO₂ results in venous serum but this method must not be used as an interchangeable alternative to calculated arterial bicarbonate in order to avoid confusion, misinterpretation of results and erroneous therapeutic decisions.