Comparison of Sodium and Potassium by Point-of-Care Arterial Blood Gas Analyzer and Venous Serum by Central Laboratory Analyser in Emergency Clinical Decision Making

Introduction: The Emergency Departments (ED) are equipped with Point-of-Care (POC) blood gas analysers (BGA) which deliver fast results on multiple parameters of arterial/venous blood. There is no consensus among ED physicians on the reliability of electrolyte results by POC Arterial Blood Gas (ABG) analysis compared to venous serum electrolyte from Central Laboratory Analyser/Auto-Analyser (CLA/AA). Aim: To compare the electrolyte(sodium and potassium) by POC arterial BGA (ABL800 Flex Radiometer) with venous electrolyte by CLA (Beckman Coulter AU 5800). Materials and Methods: This cross-sectional study was performed in the ED and Central Laboratory of the tertiary hospital from 1st July 2018 to 31st July 2019. A total of 254 critically ill adult patients with various etiologies, were enrolled in the study. The arterial and venous blood samples were collected for electrolyte measurement within a span of 15 minutes. The ABG samples, anticoagulated with liquid heparin, were processed in POC BGA. The venous samples collected in plain tubes were analysed in CLA. The results of sodium and potassium were compared by the mean, correlation coefficient, p-value, and Bland Altman Plots {95% Limit of agreement (LOA)}. Results: Out of 254 paired samples (mean age: 63±15 years), 157 (61.8%) were males and 97 (38.2%) females. The mean sodium values were 131.9±7.7 mmol/L in ABG and 132.3±7.1 mmol/L in CLA (p-value <0.0001). The mean difference was 0.4 mmol/L. The mean potassium values were 3.9±1.0 mmol/L (ABG) and 4.2±0.9 mmol/L (CLA), {p-value<0.0001}. The mean difference was 0.3 mmol/L. These differences were within the accepted range specified by the United States Clinical Laboratory Improvement Amendments. There were statistically significant strong positive correlations between the measurements of the two instruments r=0.78 for sodium and r=0.76 for potassium. The 95% LOA for sodium and potassium on both the instruments were -10.03 to 9.09 mmol/L and -1.49 to 0.97 mmol/L respectively, both wide and unacceptable. Conclusion: The arterial sodium and potassium measurements by BGA were not reliable in decision making in ED when compared to the venous serum by CLA as the 95% LOA was wide and unacceptable. Hence, sodium and potassium values by BGA alone might not be used as criteria for management without confirmation from venous serum values by CLA.