Comparing measured total carbon dioxide and calculated bicarbonate

The objective of the study is to determine the level of agreement between measured total carbon dioxide (TCO2) and calculated bicarbonate (HCO3-) in our laboratory.

TCO2 and HCO3- values of 1820 samples drawn at the same time from the patient were compared. TCO2 from venous samples was measured on Dimension RxL while HCO3- was obtained from arterial blood gas samples analyzed on Radiometer ABL 700.

The TCO2 and HCO3- values correlated well (r = 0.977, p < 0.001), with the correlation given by the equation, y = 0.986x - 0.5335. Using Bland-Altman analysis, the bias was 0.87 mmol/L (SD 1.42 mmol/L), and the limits of agreement (LOA) were -1.92 to 3.67 mmol/L. Story and Poustie's criteria were applied to study the agreement between these two methods. Based on the first criterion that the bias between TCO2 and HCO3- should be less than +/- 1 mmol/L, the results for the two methods appear to be in good agreement. The second criterion requires that the LOA between the two methods should range between a bias of +/- 2 mmol/L or a total span of 4 mmol/L; the LOA was exceeded in our study. Using the total allowable error in the Bland Altman plot also showed that the two values cannot be used interchangeably especially at the lower values.

TCO2 did not show good agreement with HCO3-. Clinicians should be aware of this discrepancy and hence should be cautious when using HCO3- for management of acid-base disorders.