Medium-term biological variation of sodium, potassium and chloride in whole blood in cohorts of healthy individuals and hemodialysis patients.

Objectives: Biological variation (BV) refers to the fluctuation of the concentration of a measurand around its homeostatic set-point. While BV has been extensively studied in healthy populations, robust data for patients with chronic diseases, particularly using whole blood measurements, remain scarce. The aim of this study was to obtain BV estimates for electrolytes in whole blood for a cohort of healthy individuals and one of hemodialysis patients (HD) and compare them with each other. Methods: A prospective BV study was conducted in 18 stable HD patients and 26 healthy individuals. Whole blood potassium, sodium and chloride concentrations were measured weekly for 12 consecutive weeks using a direct potentiometry blood gas analyser. Preanalytical conditions were standardised. BV estimates were delivered using both CV-ANOVA and Bayesian methods. Population homogeneity was assessed using the Harris-Brown heterogeneity ratio, and the index of individuality was calculated for each measurand. Results: Significant differences in BV estimates were observed between groups. BV estimates were higher in HD patients for most measurands. Within the healthy group, whole blood within-subject (CV Conclusions: This study provides the first BIVAC-compliant BV estimates for electrolytes measured in whole blood. These results highlight significant disease-related differences in BV and support the need for disease- and matrix-specific reference tools to enable personalised interpretation of electrolyte results in hemodialysis monitoring.