TOTAL CARBON-DIOXIDE MEASURED BY THE VITROS ENZYMATIC METHOD

We evaluated the performance of an enzymatic method using dry chemistr y for serum total carbon dioxide (tCO(2)) determination using a Vitros 500 analyser. Imprecision results were acceptable and the linearity w as 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 tCO(2) m ethod was unaffected by haemoglobin at all concentrations tested. Sign ificant interference was caused by bilirubin at concentrations higher than 30 mu mol/l; the addition of bilirubin lowered the apparent value s for tCO(2) dose-dependently. Serum tCO(2) results were practically t he same as those for plasma. The reference interval for venous tCO(2) concentrations in a healthy population was: 22.4-34.2 mmol/l (mean: 28 .3 mmol/l). Comparison of venous serum tCO(2) results assayed using th e Vitros method with bicarbonate (HCO3-) values calculated by blood ga s determination of pCO(2) and pH in arterial blood samples gave poor a greement, r = 0.58. The data revealed a mean difference of 5.48 +/- 3. 09 mmol/l between the tCO(2) measurements and calculated bicarbonate. This was statistically (p = 0.01) and clinically significant. We concl ude that the Vitros method provides reliable tCO(2) results in venous serum but this method must not be used as an interchangeable alternati ve to calculated arterial bicarbonate in order to avoid confusion, mis interpretation of results and erroneous therapeutic decisions.