Bioelectrical impedance vector analysis (BIVA) is a noninvasive, rapid, and relatively inexpensive technique that is widely used in the assessment of body composition and nutritional status nutritional. BIVA provides information on hydration and cell mass through the measurement of impedance (Z) and its components, resistance (Rz) and reactance (Xc). These electrical parameters are integrated into the phase angle (PhA), which has been shown to be a useful indicator of cellular integrity and nutritional status. PhA is related to the degree of hydration and tissue integrity, being a valuable prognostic marker in various pathologies, including infections, cancer and chronic diseases such as chronic renal failure.1

Resistance is inversely related to body water content, with higher resistance indicating lower hydration, while a decrease in resistance suggests overhydration. Reactance reflects cellular properties, especially the integrity of cell membranes and is directly proportional to it. A reduction in Xc may be associated with loss of mass or cell damage.2

The relevance of PhA in clinical practice has increased markedly in recent years due to its ability to reflect cellular health and its usefulness as a predictor of clinical outcomes. Recent research has shown that low PhA values are associated with worse prognosis and increased mortality in various clinical populations, including patients with cirrhosis, cancer, and those in intensive care units. In addition, numerous studies have suggested that PhA may be a marker of malnutrition and sarcopenia, providing valuable information for nutritional assessment and treatment monitoring.2, 3, 4

BIVA, and specifically PhA provides valuable information on body composition (body cell mass, i.e. nutritional status, and hydration). But in clinical practice, in patients receiving fluid therapy, it is usually recommended to perform BIVA measurements in the contralateral hemisome to which fluid therapy is administered, since the simultaneous passage of fluid could modify the hydration of that side of the body and thus alter resistance. However, there is insufficient literature addressing the influence of this aspect on the clinical significance of BIVA measurements. This knowledge gap represents an opportunity to investigate the concordance of BIVA measurements between both hemisomes in patients under fluid therapy.

Our hypothesis was that performing BIVA on the side of the body receiving intravenous fluid compared to the contralateral side has no clinical relevance. To test the hypothesis, we developed an experiment in which the agreement is analyzed with Bland–Altman plots and Passing–Block test.

The primary objective of this study was to evaluate the concordance of PhA, resistance and reactance, both standardized by height (Rz/h, Xc/h) in both hemisomes in patients receiving fluid therapy. As a secondary objective we set out to explore whether there were other covariates such as age, sex and side of the body where fluid therapy is administered, which may influence this aspect.