Endocrine Abstracts

JOINT1756

Purpose: The syndrome of inappropriate antidiuresis (SIAD) is the leading cause of hypotonic hyponatremia with preserved extracellular fluid (ECF) volume, accounting for nearly half of hospital cases, but remains a diagnosis of exclusion. Accurate classification of hyponatremia and timely treatment are essential to prevent complications and serum sodium (s-Na) overcorrection. International guidelines prioritize ECF volume classification over etiology for prompt treatment. Bioelectrical impedance vector analysis (BIVA) uses low-frequency current (50 kHz) to measure body resistance (Rz) and reactance (Xc), accurately estimating ECF. It does not rely on body geometry or tissue models and is independent of body weight, generating the Biavector plotted on a Cartesian RXc graph. This study compares the Biavector of SIAD patients to the Italian reference population to identify a potential "BIVA pattern”.

Methods: We prospectively analyzed BIVA and biochemical data from SIAD patients in two different Endocrinology Divisions in Northern Italy from November 2020 to December 2024. Inclusion criteria were a definitive SIAD diagnosis and concurrent BIVA and biochemical data. Exclusions included patients with hyponatremia from other causes, corrected s-Na after appropriate treatment, or unavailable BIVA data. BIVA confidence analysis allowed for the comparison of vector positions and hydration status across populations. Mean Biavector displacements were compared to the latest Italian reference data (Campa et al., 2023) using Hotelling’s T² test.

Results: Fifty-nine patients (56% female, age 73 [63-78.5]) were analyzed, with 63% diagnosed with senile idiopathic SIAD. In the entire cohort, the s-Na corrected for glycemia was 129 [126-131.8] mmol/l, with a p-Osm of 265 [258-273] mOsm/Kg, u-Osm of 441.6 ± 142 mOsm/Kg, u-Na of 88.5 ± 42.2 mmol/l, and copeptin levels of 4.9 [2.9-7.2] pmol/l. BIVA confidence analysis revealed a significant Biavector displacement in both female and male patients (P < 0.0001). This was characterized by a lower phase angle (PhA), possibly due to the slightly older age compared to the reference population, and a shorter Biavector, confirming a pathological yet subclinical state of overhydration.

Conclusion: Our data highlight the potential utility of BIVA analysis as an inexpensive and time-efficient tool to support the challenging differential diagnosis of patients with preserved ECF hypotonic hyponatremia due to SIAD.