Discrepancy between Measured Serum Total Carbon Dioxide Content and Bicarbonate Concentration Calculated from Arterial Blood Gases

Large differences between the concentrations of serum total carbon dioxide (TCO(2)) and blood gas bicarbonate (HCO(3)(-)) were observed in two consecutive simultaneously drawn sets of samples of serum and arterial blood gases in a patient who presented with severe carbon dioxide retention and profound acidemia. These differences could not be explained by the effect of the high partial pressure of carbon dioxide on TCO(2), by variations in the dissociation constant of the carbonic acid/bicarbonate system or by faults caused by the algorithms of the blood gas apparatus that calculate HCO(3)(-). A recalculation using the Henderson-Hasselbach equation revealed arterial blood gas HCO(3)(-) values close to the corresponding serum TCO(2 )values and clarified the diagnosis of the acid-base disorder, which had been placed in doubt by the large differences between the reported TCO(2) and HCO(3)(-) values. Human error in the calculation of HCO(3)(-) was identified as the source of these differences. Recalculation of blood gas HCO(3)(-) should be the first step in identifying the source of large differences between serum TCO(2) and blood gas HCO(3)(-).