Measured total carbon dioxide (TCO2) from venous sample and calculated bicarbonate from arterial blood gas (ABG) have shown good agreement in some studies, while conflicting results have been obtained in few other studies. The objective of this study is to compare and assess the degree of agreement between the measured TCO₂ and calculated bicarbonate and also whether they can be used interchangeably in our laboratory.

We prospectively analyzed 89 ABG samples requested for calculated bicarbonate and then measured TCO₂ from venous blood samples drawn simultaneously from the same participants between November 2016 and April 2017.

Measured TCO₂ results ranged from 5.7 to 39.9 mmol/L (mean 23.45 mmol/L), while calculated bicarbonate ranged from 9 to 40 mmol/L (mean 24.36 mmol/L). The values of TCO₂ and bicarbonate correlated well (r = 0.95, p < 0.001), with the correlation given by the equation, y = 0.884x + 3.605. The bias obtained was —0.9 mmol/L and the standard deviation (SD) was 1.62 mmol/L. The limits of agreement (LOA) were —4.1 to 2.3 mmol/L, with a span of 6.4 mmol/L. Out of the 89 values, 85 (95.05%) were within the LOA.

In majority of the cases, the calculated bicarbonate concentration from ABG showed a good correlation to the measured venous TCO₂ concentration. Despite this excellent correlation, TCO₂ did not show good agreement with calculated bicarbonate when Story and Poustie's criteria were applied, especially in cases of bicarbonate less than 20 mmol/L. Hence, clinicians should be aware of this discrepancy and be cautious when using measured TCO₂ and calculated bicarbonate interchangeably in the assessment and management of acid—base disorders, especially in patients with metabolic acidosis.

Mohan T, Kumar BV. Comparison of measured Serum Total Carbon Dioxide with calculated Bicarbonate calculated from Arterial Blood Gas Analysis. Indian J Med Biochem 2017;21(2):76-80.