The Changes in Relative Blood Volume during Dialysis Are Characterized by Ultrafiltration Rate and Predialysis Blood Test Values

<b><i>Introduction:</i></b> Continuous monitoring of relative blood volume (percentage BV) in hemodialysis (HD) is critical for determining dry weight and preventing intradialytic hypotension. However, the cause of the BV variation remains unknown. This research aimed to examine factors that influence the percentage BV. <b><i>Methods:</i></b> We devised a formula based on coefficients (“a,” “τ,” and “b”) to predict changes in percentage BV. “a” denotes a significant decrease in percentage BV in the early stages of HD. “τ” represents the transition from early to late phase of HD. “b” denotes the slope of the decrease in percentage BV in the late phase of HD. We measured the percentage BV in 18 patients with end-stage renal disease. The coefficients were estimated by fitting experimental data from patients using a least squares optimization algorithm. A correlation analysis of these parameters and patient predialysis data was performed. <b><i>Results:</i></b> Ultrafiltration rate (UFR) was found to be negatively correlated with “b” (<i>r</i> = −0.851, <i>p</i> &lt; 0.01). However, UFR was not significantly related to “a.” Predialysis serum total protein level was negatively correlated with “a” (<i>r</i> = −0.531, <i>p</i> = 0.042). Predialysis serum albumin and predialysis sodium were not significantly correlated with “a” and “τ.” Plasma osmolarity did not have a significant relationship with “a” and “τ.” <b><i>Discussion/Conclusion:</i></b> UFR influenced the decrease in percentage BV in the late phase but did not influence the decrease of percentage BV in the early phase. “a” was associated with predialysis serum total protein level but not with plasma osmolality or predialysis sodium. This implies that colloid oncotic pressure is important for plasma refilling immediately after dialysis begins. During the change of percentage BV, the decrease in the early phase of dialysis was not related to UFR, but related to other parameters, especially predialysis total protein level. A decrease in the late phase of dialysis is related to UFR.