MO660INTERRELATIONSHIP BETWEEN PROTEIN BOUND UREMIC TOXIN INDOXYL SULFATE CONCENTRATION IN BLOOD AND SPENT DIALYSATE DURING HEMODIALYSIS TREATMENT
Authors: Paats Joosep , Adoberg Annika , Arund Jürgen , Fridolin Ivo , Lauri Kai , Leis Liisi , Pilt Kristjan , Tanner Risto , Luman Merike 
Published: May 2021

Abstract

Background and Aims
Indoxyl sulfate (IS) is a representative of the protein-bound uremic retention solutes [1]. Among CKD patients, high serum levels of IS are associated with high cardiovascular and all-cause mortality – IS is linked to cardiovascular outcomes, induces acceleration of atherosclerosis and abnormal bone metabolism [2,3]. Optical monitoring of the uremic marker molecules in the spent dialysate has been proposed [4] to estimate on-line concentration and removal of uremic toxins, allowing to assess total removed solute and removal rate of uremic toxins. Although several studies have been published covering the on-line optical monitoring of the spent dialysate, there is scarce knowledge about relation between spent dialysate and blood concentrations for protein-bound uremic solutes.
The aim of this study was to evaluate the relationship between protein bound uremic toxin IS concentration in blood and spent dialysate during hemodialysis (HD) and hemodiafiltration (HDF) with different treatment settings, with the potential of evaluating uremic toxins’ levels in blood by assessing uremic toxins’ concentration in spent dialysate.

Method
22 ESKD patients (16 male and 8 female, 55±17 years) on chronic HDF were enrolled into the study (fistula N=15, graft N=7). For each patient 4 midweek dialysis sessions (length 240min, HD: N=1, Qb=200ml/min, Qd=300ml/min, 1,5m2; HDF: N=3, median (interquartile range) Qb = 298 (296-356) ml/min, Qd= 795 (500-800) ml/min, Vsubst = 21.8 (15-24.5) L, 1,8m2 and 2,2m2) were included. During each dialysis session, blood samples were taken at 0 min (start) and 240 min from the arterial blood line, and dialysate samples were taken at 7 min and 240 min from the outlet of the dialysis machine. After sample processing, serum total, serum free and spent dialysate IS concentrations were determined by HPLC. Regression analysis was carried out.

Results
Median (interquartile range) IS concentrations in blood were 10.02 (6.68 - 14.68) µmol/L for free IS, 101.33 (56.99- 125.66) µmol/L for total IS, and 3.74 (2.35- 5.93) µmol/L in dialysate at the beginning of dialysis, and 6.07 (3.58- 9.00) µmol/L, 56.70 (28.91-80.67) µmol/L, 1.94 (1.15-2.98) µmol/L at the end of dialysis, respectively. There was a strong correlation between IS concentration in blood and dialysate at the beginning and at the end of dialysis even without data normalization by treatment settings (Fig. 1), with the strongest correlation between free IS concentration and IS in dialysate at 240 min (R2 = 0,976) and at the beginning of dialysis (R2 = 0,962).
The reason for the higher correlation between free IS in blood and IS in dialysate is that only protein non-bound fraction of IS is available for removal by dialysis from blood into dialysate.

Conclusion
There is a strong correlation between IS concentrations in blood and dialysate with different treatment settings during whole dialysis. Assessment of protein bound uremic toxins’ concentration in the spent dialysate by optical sensor could thus also provide information about the concentration of uremic toxins in blood.
[1] Vanholder et al 2018; [2] Yamamoto et al 2020; [3] Barreto et al 2009; [4] Lauri et al 2019;