Calculated bicarbonate or total carbon dioxide?

To test the relationship pK' = 6.103 + log[HCO3calc] - log[HCO3meas], we used a Corning 168 blood-gas analyzer to analyze 500 blood samples for pH and PCO2, from which we calculated a value for bicarbonate. We also analyzed 500 venous blood samples, collected simultaneously, for potentiometric total carbon dioxide with the Ektachem 700 analyzer. In a similar study of 415 arterial and venous blood samples, we determined total carbon dioxide colorimetrically with the SMA 6/60 analyzer. The coefficients of determination (r2) found for the difference observed between the calculated and measured bicarbonate values vs the pK' in the two studies were 0.86 and 0.96, respectively. The results also confirmed the positive bias caused by organic acids in the Ektachem method for total carbon dioxide. Analysis of the SMA 6/60 results indicated a significant decrease of the pK' in patients classified as having a metabolic acidosis.