Re-Evaluation of Total CO₂ Concentration in Apparently Healthy Younger Adults

Abstract

The initial assessment of acid-base status is usually based on the measurement of total CO₂ concentration ([TCO₂]) in venous blood, a surrogate for [HCO₃^–]. Previously, we posited that the reference limits of serum [TCO₂] in current use are too wide. Based on studies on the acid-base composition of normal subjects, we suggested that the reference limits of serum [TCO₂] at sea level be set at 23–30 mEq/L. To validate this proposal, we queried the University of California at Los Angeles (UCLA’s) Integrated Clinical and Research Data Repository, a database containing information on 4.5 million patients seen at UCLA from 2006 to the present. Criteria for inclusion included adults (18–40 years of age), who were free of disorders that could affect acid-base balance, were not taking medications that could affect acid-base balance, and were seen for a routine medical examination or immunization in the outpatient setting. The number of individuals who met the inclusion criteria (52% female and 48% male) was 28,480, with a mean age of 28.9 ± 5.1 years. The mean serum [TCO₂] level was slightly higher in males than females, 26.6 ± 2.16 mEq/L vs. 25.0 ± 2.11 mEq/L (p < 0.05). Ninety-one percent of patient values were within the proposed 23–30 mEq/L range and 61.7% were within the 24–27 mEq/L range. These findings validate our proposal that the reference range of serum [TCO₂] in venous blood at sea level be narrowed to 23–30 mEq/L. Subjects with serum [TCO₂] outside this range might require assessment with a venous blood gas to exclude the presence of clinically important acid-base disorders.

The Continuum of Acid Stress

“…The normal range of plasma [HCO 3 − ] varies widely among clinical laboratories, with low ends as low as ≥18 mmol/L and high ends as high as ≤36 mmol/L (13), lending uncertainty as to a “low” plasma [HCO 3 − ] that is consistent with metabolic acidosis. In a large US database of apparently healthy individuals who were presumably eating typical Western diets, 91% of plasma [HCO 3 − ] determinations were between 23 and 30 mmol/L (13), supporting this narrower range for normal. These data also align with expert opinion of current guidelines that define metabolic acidosis in populations eating Western diets as plasma [HCO 3 − ] <22 mmol/L in the absence of respiratory alkalosis (14).…”