Investigation of the elimination kinetics of low and middle molecular weight uremic markers during hemodialysis treatment with the optoelectronic multispectral sensor

The elimination kinetics of conventional low-molecular weight uremic markers and advanced glycation end products (AGE products), which are considered as a possible middle molecular weight marker of uraemia in hemodialysis (HD) patients, was investigated by direct ultraviolet (UV) absorption spectroscopy of waste dialysis fluid. Light emitting diodes (LED) were exploited as quasi-monochromatic light sources in the deep UV spectral region for measuring optical transmission at the wavelengths 285 nm (the maximum of uric acid UV absorption) and 365 nm (the maximum of AGE absorption band). Optical absorption of waste dialysate was monitored during multiple HD sessions for a group of patients; the double-pool model of elimination kinetics was used for the curve fitting and approximation. It was revealed that kinetic behaviour of AGE products is almost not different comparing with uric acid and other conventional clinically proved uremic markers.


Introduction
In worldwide clinical practice most of the patients suffering from chronic renal failure (CRF) are treated with hemodialysis (HD) using various types of dialysis machines. HD therapy partially replaces normal renal function and provides elimination of middle and low-molecular weight metabolic waste products, including creatinine, phosphates, urea, uric acid, peptides, advanced glycation end products (AGE products) and many other substances. These molecules are evacuated from blood to dialysate (specially prepared fluid with electrolyte composition, which is identical to plasma) via a semi-permeable membrane by diffusion and convections mechanisms, while larger proteins and blood cells cannot travel through the pores of the membrane and remain intact [1].
The fractional urea clearance (Kt/V) is recommended by KDOQI Clinical Practice Guideline for HD Adequacy as the most preferable laboratory parameter for estimation of HD efficiency (dialysis dose). According to multiple clinical studies there is strong a correlation between patient survival and Kt/V [2]. From the other side, this approach is often considered outdated and not fully informative by many researchers [3]; alternative uremic markers have been evaluated, such as β-2-microgloubuline or Cystatin C [4].
Advanced glycation end products (AGE), which may be considered as a middle molecular weight uremic toxin, are often associated with higher risk of mortality for patients with CRF [5][6][7]. Existing biochemical methods for the determination of AGE concentration are too sophisticated for routine clinical use, while optical spectroscopic methods such fluorescence spectroscopy or absorption 2 spectroscopy in the UV and visible regions are suitable not only for the analysis of biological samples, but even for the online monitoring of AGE concentration in effluent dialysate [8].
The aim of this work is to experimentally investigate and compare the kinetics of elimination of uric acid and AGE products by UV absorption spectroscopy of effluent dialysate and assess the possibility of using AGE product for the evaluation of the dose of dialysis.

Experiment and results
An optoelectronic multispectral sensor based on UV LEDs was specially designed by Ldiamon AS (Tartu, Estonia) for the monitoring of wate dialysate optical absorption at the wavelengths 285 nm (the maximum of uric acid UV absorption) and 365 nm (the maximum of AGE absorption band). The sensor was connected to the dialysis line of a machine after a dialyzer (figure 1). Optical modules equipped with 285 nm and 365 nm deep UV LEDs are independent from each other because much longer flow-through quartz cuvette (50 mm optical length) are needed for reliable detecting of AGE products due to significantly weaker absorption of AGE compared to uric acid; two visible-blind photodetectors are used to avoid any unwanted influence of ambient light.
Reference signal (100% transmission) is recorded before the beginning of a HD treatment session when fresh dialysate is being circulated within the dialysis line of a machine. Device control and data acquisition were realized with HD Monitor 3.0 software; text files containing treatment parameters and dialysate transmittance at both analytical wavelengths, which is measured every 30 seconds, are recorded for each HD session. Measurements with the spectral sensor were carried out during more than 70 HD treatment sessions for 20 patients suffering from CRF. Time dependences of effluent dialysate transmission were recorded for each session; the results for two sessions are presented in the figure 2. It can be seen from the figure 2 that effluent dialysate transmittance at 365 nm is higher than at 285 nm despite the fact that a longer cuvette was used, otherwise both curves are very similar.
Dialysate optical absorbance at the analytical wavelengths was estimated; normalization procedure was applied to the absorption coefficients in such a way that normalized values vary in the range from 0 to 1 (extreme absorbance at the start of a treatment was used as a denominator). Normalization was  The kinetics of uremic markers elimination during hemodialysis can be analysed in the framework of double-pool kinetic model, which initially was suggested for urea, but with some limitations could be applied to other substances. This model describes both the transfer of uremic markers via a dialysis membrane and the intercompartment transfer from extracellular fluid to blood in the human body [9]: