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. ymeasured = 0.93 xcalculated +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.
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