Difference in Bicarbonate Calculation Algorithm by Manufacturer Alters Diagnosis of Acid Base Disorders
Venue: Annals of Biomedical Engineering
Year: 2019

Commercially available devices for blood gas analysis measure pH and the partial pressure of carbon dioxide (pCO2) and calculate the concentration of bicarbonate [HCO3 from them. The algorithm to calculate [HCO3 is based on the Henderson Hasselbalch equation, which can be converted to '1/2HCO 3 1/4 10dpH pKTh SCO2 pCO2' where pK and SCO2 are the equilibrium constant for carbonate species and the solubility coefficient of CO2, respectively. 1,3,4,6 Since the values of pK and SCO2 used in the algorithm differ by manufacturer, the [HCO3 provided for the same set of pH and pCO2 will differ by device. 5 Although a difference in [HCO3 affects acid base assessment, the algorithm difference by manufacturer has not been considered in the clinical practice; a common reference for [HCO3 is generally used irrespective of the difference. In this report, a possible influence of the difference on acid base assessment was examined by using clinical records. Records of venous blood gas analysis from consecutive patients managed in the department of nephrology at a general hospital in a metropolitan suburb from April 2015 to September 2017 were reviewed retrospectively. For each record, the values of [HCO3 were calculated from the measured pH and pCO2 by using the algorithm of two manufacturers (ALGO1: the algorithm of Instrumentation Laboratory Company, MA, USA and ALGO2: the algorithm of Siemens Healthineers, Erlangen, Germany) Acid base disorders were assessed by the physiological approach. The study was approved by the ethics committee of the hospital. In 439 records included, pH and pCO2 ranged from 7.10 to 7.50 and from 27 to 62 mm Hg, respectively. The [HCO3 calculated by ALGO1 [HCO3 ]ALGO1) and that calculated by ALGO2 [HCO3 ]ALGO2) ranged from 11.2 to 34.2 mmol l and from 10.7 to 33.6 mmol l, respectively. In all the records, [HCO3 ALGO1 was larger than [HCO3 ]ALGO2. There were 350 records of acidemia (pH 7.36) and 36 records of alkalemia (pH> 7.38) Values of [HCO3 in the records of acidemia and in those of alkalemia were summarized in Table 1. The diagnosis based on ALGO1 and that based on ALGO2 differed in 35 records among the 350 records of acidemia and in one record among the 36 records of alkalemia. In 22 records among the 35 records of acidemia, [HCO3 ]ALGO1 indicated absence of metabolic disorders [HCO3 25 26 mmol l) while [HCO3 ]ALGO2 indicated presence of metabolic acidosis [HCO3 25mmol l) In the other 13 records among the 35 records of acidemia, the [HCO3 ]ALGO1 indicated presence of metabolic alkalosis [HCO3 26 mmol l) while [HCO3 ]ALGO2 indicated absence of metabolic disorders. In the one record of alkalemia, [HCO3 ]ALGO1 indicated absence of metabolic disorders, while [HCO3 ]ALGO2 indicated presence of metabolic acidosis. The findings revealed that difference in the algorithm for [HCO3 would change diagnosis of acid base disorders when a common reference for [HCO3 such as 25 26 mmol l for venous blood was used. The reference for [HCO3 a calculated parameter, needs to be changed according to the algorithm. However, even when the algorithm specific references are used, the diagnosis based on ALGO1 differed from that of ALGO2 in three records among the 350 records of acidemia (Table 2) When calculated from pH 7.36 Address correspondence to Masayuki Tanemoto, Division of Nephrology, Department of Internal Medicine, International University of Health and Welfare School of Medicine, 13 1 HigashiKaigan Cho, Atami, Shizuoka 413 8790, Japan. Electronic mail: mtanemoto tky@umin.ac.jp  2019 2019)