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Patients with chronic kidney disease (CKD) are at higher cardiovascular risk than the general population. Cardiovascular diseases, vascular calcification among them, are the leading cause of death in these patients. Factors influencing vascular calcification are oxidative stress, inflammation, and accumulation of uremic toxins during CKD. Uric acid is a cardiorenal toxin that accumulates in the case of kidney malfunction. The primary therapy for replacing kidney function and removing toxins from end-stage renal disease patients is hemodialysis. Effective removal of toxins can be estimated by blood or dialysate lab analysis or optical monitoring. In this study, the authors tested a miniaturized optical sensor for monitoring uric acid levels and removal for the first time in a more extensive clinical study, including Hemodialysis (HD) and Post-dilutional online hemodiafiltration (HDF) procedures with different settings in Tallinn, Estonia. The results (Mean±SD, Lab vs. Sensor) of the uric acid concentration 57.20±34.05 vs. 57.22±33.09 µmol/L, reduction ratio 68.72±10.91 vs. 67.89±12.48 %, and total removed amount 7.00±2.10 vs. 7.33±2.29 mmol did not differ significantly from the values obtained from the clinical laboratory (p<0.05).Clinical Relevance-During this study, a miniaturized optical sensor was tested for the first time in the clinic in different dialysis settings. The results confirm that the sensor is reliable for regularly monitoring cardiorenal toxin uric acid removal during hemodialysis.

Background: Kt/V urea is the most used marker to estimate dialysis adequacy; however, it does not reflect the removal of many other uraemic toxins, and a new approach is needed. We have assessed the feasibility of estimating intradialytic serum time-averaged concentration (TAC) of various uraemic toxins from their spent dialysate concentrations that can be estimated non-invasively online with optical methods., Methods: Serum and spent dialysate levels and total removed solute (TRS) of urea, uric acid (UA), indoxyl sulphate (IS) and β2-microglobulin (β2M) were evaluated with laboratory methods during 312 haemodialysis sessions in 78 patients with four different dialysis treatment settings. TAC was calculated from serum concentrations and evaluated from TRS and logarithmic mean concentrations of spent dialysate (M ln D)., Results: Mean (± standard deviation) intradialytic serum TAC values of urea, UA, β2M and IS were 10.4 ± 3.8 mmol/L, 191.6 ± 48.1 µmol/L, 13.3 ± 4.3 mg/L and 82.9 ± 43.3 µmol/L, respectively. These serum TAC values were similar and highly correlated with those estimated from TRS [10.5 ± 3.6 mmol/L ( R 2  = 0.92), 191.5 ± 42.8 µmol/L ( R 2  = 0.79), 13.0 ± 3.2 mg/L ( R 2  = 0.59) and 82.7 ± 40.0 µmol/L ( R 2  = 0.85)] and from M ln D [10.7 ± 3.7 mmol/L ( R 2  = 0.92), 191.6 ± 43.8 µmol/L ( R 2  = 0.80), 12.9 ± 3.2 mg/L ( R 2  = 0.63) and 82.2 ± 38.6 µmol/L ( R 2  = 0.84)], respectively., Conclusions: Intradialytic serum TAC of different uraemic toxins can be estimated non-invasively from their concentration in spent dialysate. This sets the stage for TAC estimation from online optical monitoring of spent dialysate concentrations of diverse solutes and for further optimization of estimation models for each uraemic toxin., Competing Interests: A.F. has received consultancy or speaker fees from Otsuka, AstraZeneca, Vifor Pharma and Alnylam. A.O. is the former CKJ Editor-in-Chief and has received grants from Sanofi and consultancy or speaker fees or travel support from Advicciene, Astellas, AstraZeneca, Amicus, Amgen, Fresenius Medical Care, GlaxoSmithKline, Bayer, Sanofi-Genzyme, Menarini, Kyowa Kirin, Alexion, Idorsia, Chiesi, Otsuka, Novo Nordisk and Vifor Fresenius Medical Care Renal Pharma and is director of the Catedra Mundipharma-UAM of diabetic kidney disease and the Catedra AstraZeneca-UAM of chronic kidney disease and electrolytes. V.M.P.-G. has received grants from Catedra Mundipharma-UAM and Catedra AstraZeneca-UAM and consultancy or speaker fees or travel support from Kyowa Kirin, Alexion and Otsuka AstraZeneca and Sanofi-Genzyme. M.S. has received grants from Hansa Biopharma and consulting fees from Hansa Biopharma, Chemocentryx, AstraZeneca and Vifor Pharma. All other authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results., (© The Author(s) 2022. Published by Oxford University Press on behalf of the ERA.)

Optical online methods are used to monitor the haemodialysis treatment efficiency of end stage kidney disease (ESKD) patients. The aim of this study was to analyse the effect of the administration of UV-absorbing drugs, such as paracetamol (Par), on the accuracy of optical monitoring the removal of uremic toxins uric acid (UA) and indoxyl sulfate (IS) during standard haemodialysis (HD) and haemodiafiltration (HDF) treatments. Nine patients received Par in daily dosages 1−4 g for 30 sessions. For 137 sessions, in 36 patients the total daily dosage of UV-absorbing drugs was less than 500 mg, and for 6 sessions 3 patients received additional UV-absorbing drugs. Par administration slightly affected the accuracy of optically assessed removal of UA expressed as bias between optically and laboratory-assessed reduction ratios (RR) during HD but not HDF employing UV absorbance of spent dialysate (p < 0.05) at 295 nm wavelength with the strongest correlation between the concentration of UA and absorbance. Corresponding removal of IS based on fluorescence at Ex280/Em400 nm during HD and HDF was not affected. Administration of UV-absorbing drugs may in some settings influence the accuracy of optical assessments in spent dialysate of the removal of uremic solutes during haemodialysis treatment of ESKD patients.

Optical monitoring of spent dialysate has been used to estimate the removal of water-soluble low molecular weight as well as protein-bound uremic toxins from the blood of end stage kidney disease (ESKD) patients. The aim of this work was to develop an optical method to estimate the removal of β2-microglobulin (β2M), a marker of middle molecule (MM) uremic toxins, during hemodialysis (HD) treatment. Ultraviolet (UV) and fluorescence spectra of dialysate samples were recorded from 88 dialysis sessions of 22 ESKD patients, receiving four different settings of dialysis treatments. Stepwise regression was used to obtain the best model for the assessment of β2M concentration in the spent dialysate. The correlation coefficient 0.958 and an accuracy of 0.000 ± 0.304 mg/L was achieved between laboratory and optically estimated β2M concentrations in spent dialysate for the entire cohort. Optically and laboratory estimated reduction ratio (RR) and total removed solute (TRS) of β2M were not statistically different ( p > 0.35). Dialytic elimination of MM uremic toxin β2M can be followed optically during dialysis treatment of ESKD patients. The main contributors to the optical signal of the MM fraction in the spent dialysate were provisionally identified as tryptophan (Trp) in small peptides and proteins, and advanced glycation end-products.

In this study, simultaneous removal assessment of marker molecules from three uremic toxin groups was performed during different hemodialysis treatment modalities using optical characteristics of spent dialysate. Results from optical measurements were compared with the results from chemical laboratory. Ten chronic dialysis patients, mean age 59 ± 15 years, were included in the study during 40 hemodialysis sessions. Low-flux hemodialysis (HD), high-flux hemodialysis (HF), and postdilutional online hemodiafiltration (HDF) with different settings were used. The reduction ratio (RR) and total removed solute (TRS) of three uremic solutes were determined: small molecular weight urea, middle molecular β2-microglobulin (B2M), and protein-bound indoxyl sulfate (IS). Concentrations of these solutes in the spent dialysate were measured by laboratory (lab) and optical (opt) methods, in the serum by laboratory methods, and calculated RR values in percentage were compared accordingly. Total removed solute was obtained from the total dialysate collection (TDC) using lab and opt methods. The highest RR values were found for urea and B2M, and the lowest for IS. The difference between RR of lab and opt results estimated as mean accuracy (BIAS) was ≤8.1% for all three solutes. Good correspondence between TRS lab vs. opt was achieved, resulting in strong linear correlation values R from 0.727 for urea to 0.971 for IS. Accuracy for TRS values as BIAS ± standard error (SE), comparing lab vs. opt, showed no statistical difference for any of the observed uremic solutes (P > 0.05). The accuracy of the optical method was not influenced by the dialysis modality (HD, HF, and HDF).

Tryptophan is an essential dietary amino acid that originates uremic toxins that contribute to end-stage kidney disease (ESKD) patient outcomes. We evaluated serum levels and removal during haemodialysis and haemodiafiltration of tryptophan and tryptophan-derived uremic toxins, indoxyl sulfate (IS) and indole acetic acid (IAA), in ESKD patients in different dialysis treatment settings. This prospective multicentre study in four European dialysis centres enrolled 78 patients with ESKD. Blood and spent dialysate samples obtained during dialysis were analysed with high-performance liquid chromatography to assess uremic solutes, their reduction ratio (RR) and total removed solute (TRS). Mean free serum tryptophan and IS concentrations increased, and concentration of IAA decreased over pre-dialysis levels (67%, 49%, -0.8%, respectively) during the first hour of dialysis. While mean serum total urea, IS and IAA concentrations decreased during dialysis (-72%, -39%, -43%, respectively), serum tryptophan levels increased, resulting in negative RR (-8%) towards the end of the dialysis session ( p < 0.001), despite remarkable Trp losses in dialysate. RR and TRS values based on serum (total, free) and dialysate solute concentrations were lower for conventional low-flux dialysis ( p < 0.001). High-efficiency haemodiafiltration resulted in 80% higher Trp losses than conventional low-flux dialysis, despite similar neutral Trp RR values. In conclusion, serum Trp concentrations and RR behave differently from uremic solutes IS, IAA and urea and Trp RR did not reflect dialysis Trp losses. Conventional low-flux dialysis may not adequately clear Trp-related uremic toxins while high efficiency haemodiafiltration increased Trp losses.

Aim: In this work we estimated the contribution of the fluorescence of 4-pyridoxic acid (4-PA) to the total fluorescence of spent dialysate with the aim of evaluating the on-line monitoring of removal of this vitamin B-6 metabolite from the blood of patients with end-stage renal disease (ESRD)., Methods: Spectrofluorometric analysis of spent dialysate, collected from hemodialysis and hemodiafiltration sessions of 10 patients receiving regularly pyridoxine injections after dialysis treatment, was performed in the range of Ex/Em 220-500 nm. 4-PA in dialysate samples was identified and quantified using HPLC with fluorescent and MS/MS detection., Results: Averaged HPLC chromatogram of spent dialysate had many peaks in the wavelength region of Ex320/Em430 nm where 4-PA was the highest peak with contribution of 42.2±17.0% at the beginning and 47.7±18.0% in the end of the dialysis. High correlation (R = 0.88-0.95) between 4-PA concentration and fluorescence intensity of spent dialysate was found in the region of Ex310-330/Em415-500 nm, respectively., Conclusion: 4-PA elimination from the blood of ESRD patients can be potentially followed using monitoring of the fluorescence of the spent dialysate during dialysis treatments., Competing Interests: The authors declare that they have no conflict of interest.

The aim of this study was to evaluate the contribution and removal dynamics of the main fluorophores during dialysis by analyzing the spent dialysate samples to prove the hypothesis whether the fluorescence of spent dialysate can be utilized for monitoring removal of any of the protein bound uremic solute. A high performance liquid chromatography system was used to separate and quantify fluorophoric solutes in the spent dialysate sampled at the start and the end of 99 dialysis sessions, including 57 hemodialysis and 42 hemodiafiltration treatments. Fluorescence was acquired at excitation 280 nm and emission 360 nm. The main fluorophores found in samples were identified as indole derivatives: tryptophan, indoxyl glucuronide, indoxyl sulfate, 5-hydroxy-indoleacetic acid, indoleacetyl glutamine, and indoleacetic acid. The highest contribution (35 ± 11%) was found to arise from indoxyl sulfate. Strong correlation between contribution values at the start and end of dialysis (R2 = 0.90) indicated to the stable contribution during the course of the dialysis. The reduction ratio of indoxyl sulfate was very close to the decrease of the total fluorescence signal of the spent dialysate (49 ± 14% vs 51 ± 13% respectively, P = 0.30, N = 99) and there was strong correlation between these reduction ratio values (R2 = 0.86). On-line fluorescence measurements were carried out to illustrate the technological possibility for real-time dialysis fluorescence monitoring reflecting the removal of the main fluorophores from blood into spent dialysate. In summary, since a predominant part of the fluorescence signal at excitation 280 nm and emission 360 nm in the spent dialysate originates from protein bound derivatives of indoles, metabolites of tryptophan and indole, the fluorescence signal at this wavelength region has high potential to be utilized for monitoring the removal of slowly dialyzed uremic toxin indoxyl sulfate.

The aim of this study was to assess removal dynamics of paracetamol (PAR), as an extraordinary chromophore in spent dialysate, upon the optical monitoring of dialysis of end-stage renal disease patients with inflammation complications. Seven dialysis sessions of different patients were followed to whom PAR was used as a pain reliever or antipyretic. Spent dialysate was sampled hourly and analyzed using HPLC with MS/MS and UV detection. Quantitative calculations were made on the basis of the peak areas on the chromatograms at 280 nm for uric acid (UA) and 254 nm for PAR and its metabolites (PAR-M). Peaks of UA, PAR, PAR-glucuronide, and PAR-sulphate were identified on the basis of specific mass spectra. Removal of PAR was found to be proportional to that of uric acid if intake of the drug by patient occurred half a day before dialysis. But disturbances of the UV-absorbance curves at 280 nm were observed related to rise of UA concentration in spent dialysate when PAR was taken by patients in the course of dialysis. The mechanism of such relation remains unknown. It was concluded that possible benefits and risks of treatment of uremic patients with paracetamol-containing drugs may need to be reassessed.

The aim of this study was to explore the possibility to determine the amount of total removed Indoxyl Sulphate (TR&#95;IS) during dialysis session, an optical method utilizing absorbance and fluorescence spectral data of the spent dialysate was used. Eight uremic patients from Linköping, Sweden and 10 from Tallinn, Estonia, were studied during dialysis treatments. Dialysate samples were taken during each treatment and analyzed at a laboratory. Fluorescence and absorbance spectra of the spent dialysate were measured with spectrofluorophotometer and spectrophotometer. The spectral values were transformed into IS concentration using multiple linear regression model from the total material noted as optical method (Opt). IS concentration was estimated using high-performance liquid chromatography (HPLC) method as a reference. TR&#95;IS values were calculated. Achieved results were compared regarding mean values and SD and collated with the amount of total removed urea value (TR&#95;Urea) for the same dialysis procedures. Mean TR value ± SD (mg) for urea was 28 947 ± 9 241; TR for IS was 151.4 ± 87.3 estimated by HPLC and 149.4 ± 84.9 estimated by Opt. The TR&#95;IS values were not significantly different (p ≤ 0.05). This study indicates, that it is possible to estimate TR&#95;IS using only spectral values of the spent dialysate and the parameter can be used for quantifying the elimination of protein bound uremic toxins during the dialysis procedure.

The aim of this work was to evaluate the contributions of the main chromophores to the total UV absorbance of the spent dialysate and to assess removal dynamics of these solutes during optical on-line dialysis dose monitoring. High performance chromatography was used to separate and quantify UV-absorbing solutes in the spent dialysate sampled at the start and at the end of dialysis sessions. Chromatograms were monitored at 210, 254 and 280 nm routinely and full absorption spectra were registered between 200 and 400 nm. Nearly 95% of UV absorbance originates from solutes with high removal ratio, such as uric acid. The contributions of different solute groups vary at different wavelengths and there are dynamical changes in contributions during the single dialysis session. However, large standard deviation of the average contribution values within a series of sessions indicates remarkable differences between individual treatments. A noteworthy contribution of Paracetamol and its metabolites to the total UV absorbance was determined at all three wavelengths. Contribution of slowly dialyzed uremic solutes, such as indoxyl sulfate, was negligible.