Re-Evaluation of the Normal Range of Serum Total CO2 Concentration

A reliable determination of blood pH, PCO 2 , and [HCO 3 − ] is necessary for assessing the acid-base status of a patient. However, most acid-base disorders are first recognized through abnormalities in serum total CO 2 concentration ([TCO 2 ]) in venous blood, a surrogate for [HCO 3 − ]. In screening patients on the basis of serum [TCO 2 ], we have been concerned about the wide limits of normal for serum [TCO 2 ], 10–13 mEq/L, reported by many clinical laboratories. Indeed, we have encountered patients with serum [TCO 2 ] values within the lower or upper end of the normal range of the reporting laboratory, who subsequently were shown to have a cardinal acid-base disorder. Here, we present a patient who had a serum [TCO 2 ] within the lower end of the normal range of the clinical laboratory, which resulted in delayed diagnosis of a clinically important “hidden” acid-base disorder. To better define the appropriate limits of normal for serum [TCO 2 ], we derived the expected normal range in peripheral venous blood in adults at sea level from carefully conducted acid-base studies. We then compared this range, 23 to 30 mEq/L, to that reported by 64 clinical laboratories, 2 large commercial clinical laboratories, and the major textbook of clinical chemistry. For the most part, the range in the laboratories we queried was substantially different than that we derived and that published in the textbook, with some laboratories reporting values as low as 18–20 mEq/L and as high as 33–35 mEq/L. We conclude that the limits of values of serum [TCO 2 ] reported by clinical laboratories are very often inordinately wide and not consistent with the range of normal expected in healthy individuals at sea level. We suggest that the limits of normal of serum [TCO 2 ] at sea level be tightened to 23–30 mEq/L. Such correction will ensure recognition of the majority of “hidden” acid-base disorders.