Research Viewpoints What is the Normal Range of Values For Venous Serum Total CO2?

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Kraut J & Madias N: Re-Evaluation of Total CO2 Concentration in Apparently Healthy Younger Adults. Am J Nephrol 2018;48:15–20

Surprisingly, the normal limits of venous serum total CO2 (tCO2; a surrogate for serum HCO3- concentration) in adults are not well established.

Kraut and Madias set out to correct this deficiency by examining the tCO2 levels of venous serum in 28,480 apparently normal young adults (ages 18 – 40 years; mean age = 28.9 ± 5.1 years). 91 % of the values fell between 23 – 30 mmol/l – a span of only 8 mmol/l between high and low normal values. Many clinical laboratories report a wider range of values with lower limits of 18 – 25 mmol/l and upper limit of normal of 26 – 35 mmol/l. Interestingly, in this study there was a small but consistent gender difference in the normal values for serum tCO2; slightly higher values in males than females (about a 1.6 mmol/l difference on average). Pre-menopausal females may experience greater alveolar ventilation than males due to the effects of progesterone. One might expect these gender differences to disappear after menopause and to be aggravated with pregnancy. The influence of diet (e.g. vegan vs omnivore) could not be assessed in this study, but one might expect somewhat higher levels of tCO2 in vegans.

The main message of this paper is that when venous serum tCO2 levels are < 23 or > 30 mmol/l, an arterial blood gas measurement should be undertaken to define the nature of the acid-base disturbance, unless the cause and mechanism of the abnormality is obvious.