Discordance between measured and calculated total carbon dioxide.

Recent studies on the agreement and correlation between measured and calculated total CO2 (TCO2) have yielded conflicting results. Pre-analytical variation could have been partially responsible. While keeping such variables at an absolute minimum, we found excellent correlation (r = 0.98) in 88 samples, with only a small variation in agreement between measured and calculated TCO2 values (SD = 1.1 mmol/L), which could be a function of variation in apparent pK (pK'). A subsequent evaluation of 913 consecutive samples, routinely analyzed, yielded similar results. These results suggest that some of the discrepancies reported in the literature could be ascribable to differences in sample types and sample handling. Rigid control of pre-analytical procedures is therefore a prerequisite in studies on this topic. The two methods were found to agree over a wide range of values, such that either of them could be used to evaluate clinical acid-base status accurately.