Estimation of Excess Extracellular Fluid Volume Using Blood Volume Monitoring: A Comparison With Bioimpedance...

Excess extracellular fluid volume (excess V_ECF ) is associated with poor prognosis in dialysis patients. Although bioimpedance spectroscopy is widely used to assess excess V_ECF  , it requires specialized equipment. The aim of this study was to develop a novel method to estimate excess V_ECF based on blood volume monitoring data.

We constructed a mathematical model to estimate excess V_ECF using the relative blood volume at the end of dialysis. The estimated excess V_ECF at the end of dialysis was compared with the bioimpedance spectroscopy-derived excess V_ECF measured immediately after the same dialysis session. The correlation and agreement between the two methods were evaluated.

A significant correlation was observed between blood volume-based and bioimpedance spectroscopy-derived excess V_ECF (r ² = 0.7526, p < 0.0001). Bland-Altman analysis showed a fixed bias, indicating that blood volume-based excess V_ECF systematically overestimated bioimpedance spectroscopy-derived excess V_ECF at the end of dialysis, but no proportional bias was observed (mean difference = +0.86 L/m²; 95% limits of agreement = -1.10 to +2.82 L/m²; r = 0.046).

Possible sources of estimation error in blood volume-based excess V_ECF include the assumption of isotonicity during dialysis and variability in the f-cell ratio.

The proposed blood volume-based method for estimating excess V_ECF demonstrated a significant correlation with bioimpedance spectroscopy-derived values and may serve as a useful tool for volume assessment in dialysis patients. However, further validation is needed before it can be considered a substitute for bioimpedance spectroscopy as a standard method.