Comparison of whole blood potentiometry using arterial blood gas analyser at the intensive care unit with laboratory-based serum sodium and potassium estimation in critically ill patients.

Background: Electrolyte measurement in critically ill patients is a crucial investigation guiding the management of the patients. Accuracy of the results and rapid turnaround time are hence very important in the analysis of electrolytes. In our study, we aim to compare two methods of electrolyte concentration measurement by direct ion-selective electrodes (direct ISEs) using patient whole blood sample in the intensive care unit (ICU) and indirect ion-selective electrodes (indirect ISEs) using serum sample in laboratory. Methods: A prospective observational study was conducted on 142 paired samples that were obtained from patients admitted to the ICU. The arterial whole blood was analysed in an arterial blood gas analyser located in the ICU for analysis of electrolytes by Direct-ISE, whereas the blood collected for serum electrolytes was promptly transported to the hospital laboratory where serum was separated and analysed in an automated analyser using indirect ISE. Statistical analysis was done by using the t-Test and Bland-Altman plots. The Haemolysis Icteric Lipemia Index given by the laboratory analyser and serum total protein value were also recorded. Results and conclusion: In our study conducted on 142 paired samples for electrolyte analysis in critically ill patients by direct ISE and indirect ISE, we observed that there is a statistically significant difference between the average of measurements by the two methods. This difference is more significant at lower sodium levels and normal potassium levels. A larger sample size is required to shed more light on the clinical implications in the tertiary-care setup where the study was organised.