Failure of the serum CO2 determined by automation to estimate the plasma bicarbonate

In the daily care of pulmonary disease patients, we have often encountered large differences in the bicarbonate value calculated from blood gas determination pCO2 and pH, and the serum CO2 obtained from the Dupont Automatic Clinical Analyzer (ACA). Realizing that many clinicians utilize the automated measurement of serum CO2 ([CO2]s) in place of blood gas determinations, we felt it necessary to analyze paired sets of ACA [CO2]s and blood gas bicarbonate values ([HCO3-]p) for variations which might lead to clinically significant differences in interpretation. A study of 1,841 patient samples, matched by date and within +/- 2 hours of sampling, and supported by a thorough chart review of 100 randomly selected patients, indicates that a statistically significant difference, which is unrelated to apparent changes in the patient clinical status, does exist. Evaluation of pK changes due to temperature failed to correct these differences in serum CO2 and plasma bicarbonate. Based on the limited information available in the literature, as well as the strict quality control measures utilized in the measuring of [CO2]s using the ACA method and the measurement of bicarbonate by blood gas determination, we have concluded that the differences in the [CO2]s and [HCO3-]p can only be related to erroneous assumptions about the method of measuring [CO2]s by the ACA or other automated methods as well as manual techniques of back titration.