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

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.