Estimating Arterial Bicarbonate (HCO3) from Venous Blood Using the Kidney Electrolyte Panel

Background: Arterial blood gas (ABG) analysis remains the gold standard for diagnosis and management of acid-base disorders. Venous blood is much more easily obtainable but there is a need for reliable data to convert venous to arterial data. At last year’s ASN meeting we presented results from 7470 samples where blood pCO2 and pH had been measured at the same time for pH and pCO2. We now report on how to best calculate arterial HCO3 from venous blood using a subset of those samples (n=1425) where total venous CO2 had also been obtained at the same time. Methods: Data were extracted from the medical records of totally de-identified subjects. A total of 1425 samples where it was verified that not only the sampling for venous and arterial blood gas was performed at the same time but that there were also concurrent measurements of total CO2 from an electrolyte panel All measurements were done from samples delivered at the laboratories of Northwestern Medicine over a period of 7 years. The information was provided to us by an independent data analyst and Linear regression and Bland Altman plots were used. Results: The relationship between venous HCO3 and arterial HCO3 was strong (r=0.89). From the equation defining this relationship venous HCO3 was corrected and plotted against the total CO2 obtained from the venous electrolyte panel. The correlation was also strong (r=0.84) (figure) indicating that the total CO2 can be used to replace arterial bicarbonate in the Henderson Hasselbalch equation. Conclusion: Using a large database with concurrent measurements of arterial and venous blood accurate estimations of arterial bicarbonate could be made from the venous total CO2 in the renal electrolyte panel. This information has been integrated in an App that allows conversion of venous to arterial blood gas data.