**1. Introduction**

142 Progress in Hemodialysis – From Emergent Biotechnology to Clinical Practice

Ward, R. A., Schmidt, B., & Hullin, J. (2000). A comparison of on-line hemodiafiltration and

Weber, V., Linsberger, I., & Rossmanith, E. (2004). Pyrogen transfer across high- and low-

Weissinger, E. M., Kaiser, T., & Meert, N. (2004). Proteomics: a novel tool to unravel the

Woods, H. F. & Nandakumar, M. (2000). Improved outcome for haemodialysis patients

Yamamoto, K., Hiwatari, M., & Kohori, F. (2005). Membrane fouling and dialysate flow

No.12, (Dec), pp.(2344-2350), 1046-6673,1533-3450(e)

pp.(3068-3077), 0931-0509,1460-2385(e)

42), 0931-0509,1460-2385(e)

pp.(198-205), 1434-7229

564X,1525-1594(e)

high-flux hemodialysis: a prospective clinical study. *J Am Soc Nephrol,* Vol.11,

flux hemodialysis membranes. *Artif Organs,* Vol.28, No.2, (Feb), pp.(210-217), 0160-

patho-physiology of uraemia. *Nephrol Dial Transplant,* Vol.19, No.12, (Dec),

treated with high-flux membranes. *Nephrol Dial Transplant,* Vol.15 Suppl 1, pp.(36-

pattern in an internal filtration-enhancing dialyzer. *J Artif Organs,* Vol.8, No.3,

Uremic toxicity is described as a clinical picture resulting from impaired renal elimination and accumulation of uremic toxins in the body. Uremic toxins can be classified as small water-soluble compounds, middle molecules and protein-bound compounds. A long list of possible uremic toxins has been identified in recent decades. Under normal conditions these compounds are excreted by healthy kidneys. If kidney function fails, waste products accumulate in the blood and in the body. Dialysis treatment replaces some kidney functions through diffusion (waste removal) and ultrafiltration of fluid across a semi-permeable membrane, which is a thin layer of material with holes or pores of various sizes. A deeper understanding about the accumulation and removal mechanisms of the retained solutes during care of renal insufficiency is needed (Eloot, 2008; Eloot, 2007). This understanding would be especially informative with respect to predicting the mode of action of uremic toxins and their specific role in complications associated with dialysis or ureamia, but also with cardiovascular disease and inflammation (Vanholder, et al., 2008; Vanholder, 2003). The methods contributing to the identification, characterisation and evaluation of uremic retention solutes could be assessed in order to ensure dialysis adequacy and quality (Vanholder, 2005). The choice of the correct concentration of potential uremic toxins is still an unresolved issue (Vanholder, 2003). In everyday clinical practice, uremic components are not examined due to the measurement of most uremic components using the available laboratory methods being difficult and complex. A number of standard biochemical techniques are used in clinical laboratories, but there is no universal methodology. In addition, some chromatographic methods have been developed which explore uremic retention solutes.

Dialysis efficiency and quality has been an important issue accompanied by optimisation and the best outcome of the treatment of chronic end stage renal disease for many years. In connection to this, online monitoring of the dialysis dose has been suggested as an effective way of improving treatment quality. The concept of online monitoring is based on the realtime measurement of chemical signals coming from the patient. This enables the early

Optical Dialysis Adequacy Monitoring:

Small Uremic Toxins and Contribution to UV-Absorbance Studied by HPLC 145

spectrometer is far more complicated because of the underlying optical principle, in which interference or diffraction is utilised (Jensen, 2002; Cho, 2008). The UV method is more straightforward and is not as demanding considering the source and detector characteristics and other technological modules. The UV method has recently been commercialised as a monitoring tool for dialysis dose in terms of the urea-based parameters, Kt/V and URR. Validation studies of the system have shown that the results with UV technology are

An optical dialysis online monitoring system utilising UV absorbance has been developed for the continuous monitoring of the toxins eliminated by the spent dialysate. This system represents a spectrophotometer connected to the fluid outlet of the dialysis machine. All spent dialysate passes through a specially designed optical cuvette. The transmitted light intensity of the spent dialysate is measured. All substances – chromophores – absorbing the UV-radiation at a particular wavelength, give the total online UV absorbance curve. The

The obtained UV absorbance values are processed and presented on the computer screen by a PC incorporated in the spectrophotometer using special software. The absorbance A of a solution, obtained by the spectrophotometer using the pure dialysate as the reference

*<sup>I</sup> <sup>A</sup>*

where Ir is the intensity of transmitted light through the reference solution (pure dialysate) and Ir+s is the summated intensity of transmitted light through the reference solution containing the solutions under study (pure dialysate + waste products from the blood). The absorbance is measured in arbitrary units (a.u.). A sampling frequency at two samples per minute is usually sufficient, but can be much higher (e.g. if a more detailed curve is desired

*r s*

(2)

*I*

schematic clinical set-up of online monitoring experiments is shown in Figure 1.

indistinguishable from blood based Kt/V (Castellarnau, 2010).

Fig. 1. Clinical experimental set-up and sample analysis

log *<sup>r</sup>*

solution, is determined as:

or noise reduction is necessary).

recognition of signs of intolerance and allows for early intervention. Despite the first online monitors of dialysis doses being available today, it is essential to monitor the patient's condition during the dialysis session and, if necessary, change some conditions of dialysis treatment (e.g. time) more specifically considering uremic toxins with various characteristics and elimination profiles.

The aim of this chapter is to describe and assess performance of optical dialysis adequacy monitoring technique related to the removal of uremic retention solutes during dialysis. The assessment is based on the high performance liquid chromatography (HPLC) profiles of the serum and the spent dialysate connected to the origin of the cumulative and integrated UV absorbance arising from the contribution of uremic retention solutes - chromophores, among them probably several uremic toxins.
