Protein-bound uremic toxins (PBUT), linked with the progression of renal disease and cardiovascular disease, accumulate in ESKD patients. Several recent studies have attempted to use albumin binding competitors or displacers for enhanced PBUT elimination. However, careful monitoring of administrating displacers is needed to avoid adverse clinical effects for the patients. Some of the potential displacers (e.g. Furosemide) absorb UV-light, offering an opportunity to monitor the chemical displacer treatment optically. The aim of this study was to investigate whether a displacer-chromophore (DC) administration into the blood line can be monitored optically on-line in the spent dialysate during a hemodialysis (HD) treatment. 10 ESKD patients in total (4 from Linköping, Sweden and 6 from Tallinn, Estonia) during a single post-HDF therapy (Vsubst 15 L, Qb=300 mL/min, Qd=800 mL/min, filter FX1000 2.2 m2) were included into the study. Infusion of a displacer (Furosemide, 10 mL 10 mg/mL solution) with an infusion pump upstream from the arterial port of the dialyzer was applied during 7 minutes, after about 75 minutes from the start of the treatment (Fig 1, starting at moment a). Spent dialysate samples were collected at the dialysate outlet port of the HD machine before, during and after Furosemide injection (Fig 1, time moments a, b and c) for uric acid (UA), a major chromophore in spent dialysate, measurement by HPLC. The optical on-line monitoring of spent dialysate was performed by a dialysis sensor (OLDIAS) connected to the spent dialysate outlet of HD machine. For comparison, absorbance signal and concentrations of UA in the spent dialysate as being without the effect of the DC were predicted (Fig 1, dotted line) by the logarithmic mean equation from respective values before and after furosemide injection (Fig 1, time moments a and c). The absorbance spectrum of Furosemide was measured with a spectrophotometer. The optical signal amplitude at 280 nm in spent dialysate clearly increased during a bolus of the displacer was injected into the blood line during a dialysis treatment (Fig 1). The absolute increase of absorbance in spent dialysate was very similar for all patients, being 0,056±0,005 a.u. (mean±SD) and the relative increase was 9.15±1.93% (mean±SD). Meanwhile, the concentration of UA was not influenced by Furosemide, having a very low difference of -0,20±1,13%, comparing actual and predicted concentrations. The contribution of PBUT to the absorbance signal is known to be negligible. As Furosemide has one of its peak absorbances very close to 280 nm (Fig 2) and concentration of the major chromophore UA was not influenced by DC, the increase in the absorbance can be mainly related to the DC injection. Administration of the displacer-chromophore into the blood line during a dialysis treatment can be monitored optically on-line, and the time and duration of the injection can be exactly located and recorded for the further analysis or for the treatment log.