**7. Conclusions**

156 Progress in Hemodialysis – From Emergent Biotechnology to Clinical Practice

different (p < 0.05) from the RR of serum uric acid and urea. The similar removal of urea and uric acid makes it possible to use other components and methods to monitor urea reduction during a single hemodialysis session. At the same time, the RR of creatinine is statistically different (p < 0.05) from the RR of all HPLC peaks in the serum at 280 nm, but not different from that of 254 nm. The RR for online UV absorbance is lower compared to urea. Considering that RR (URR) is correlated to Kt/V (NKF-DOQI, 2006), this tendency is reported earlier, as the dialysis dose estimated by online UV absorbance was lower than Kt/V urea (Uhlin, 2003). The difference between the RR of all HPLC peaks in serum and online UV absorbance measurement (at 280 nm) in spent dialysate could be due to different chromophores in the serum and spent dialysate and because the serum was collected before and the dialysate sample 10 minutes after the start of dialysis. Moreover, the different binding of individual uremic retention solutes to serum proteins may modify percentage concentration changes of individual solutes in the course of haemodialytic treatment (Vanholder, 1992), supported by observations of decreased drug/protein interactions in

The correlation analysis also provides additional insights into the removal characteristics of solutes and UV absorbance monitoring (Table 2). The RR of uric acid has the highest correlation for RR at 280 nm in both serum and spent dialysate, but not at 254 nm in serum. The explanation is the highest contribution of uric acid to UV absorbance compared to other chromophores at 280 nm (Figures 5 and 6). However, there are several other strong contributions from other compounds beside uric acid at 254 nm, and therefore the correlation is lower. The outcome in Table 2 is confirmed by comparing the millimolar extinction coefficients versus wavelengths for uric acid (Vasilevsky, 2005). A higher value of the extinction coefficient corresponds to the higher correlation for RR of uric acid at 280 nm. The RR of urea is more related to RR at 280 nm both in serum and spent dialysate, but less so at 254 nm in serum (Fridolin, 2003). This means that relatively good correlation between the RR of UV absorbance and a particular solute may be achieved when the removal rate of a non-absorbing solute (e.g. urea) is similar to that of UV-absorbing substances during haemodialysis. This is also confirmed by very good correlation between several small molecular weight waste products and UV absorbance (Fridolin, 2002) and similar concentration changes during dialysis for several azotemic markers (e.g. urea, creatinine and uric acid) (Vanholder, 1992). The dominance of small molecular weight waste products among chromophores in serum and spent dialysate can be concluded because the number of detected HPLC peaks is not significantly different for serum filtered with a filter in cut-off 3 kDa and 70 kDa. Furthermore, it seems that the UV-absorbing solutes can be subject to similar corrections regarding distribution volume and intercompartmental equilibration rates, similar to urea, although not with exactly the same distribution and equilibration intercompartmental rates in the body as urea. This makes it possible to estimate the delivered dialysis dose in terms of Kt/V by monitoring UV absorbance in spent dialysate

The RR of creatinine demonstrates a high correlation for RR at 280 nm in both serum and spent dialysate. The reason could be similar removal of creatinine and other chromophores at 280 nm, where creatinine does not contribute significantly to UV absorbance (Figure 5). As previous studies have shown, and as is confirmed by this work, the HPLC is a method which has its own place for the detection of uremic solutes in biological solutes. This is an effective method of studying accumulated metabolites in patients' blood and removed in dialysis. Identification of these metabolites gives us the opportunity to understand the

uremic serum (De Smet, 1999).

online (Uhlin, 2003).

This chapter contributes new information about the removal of uremic retention solutes during hemodialysis and the origin of the optical dialysis adequacy monitoring signal. The relationship between characteristics of the online UV absorbance curve measured during dialysis and the identified HPLC peaks in spent dialysate was investigated. It was demonstrated that the absorbance signal reflects the contribution of several UV-absorbing compounds in spent dialysate, with the strongest influence coming from the low-molecularweight water-soluble non-protein bound compounds. Moreover, UV absorbance behaves more like small water-soluble non-protein-bound solutes than small protein-bound solutes. Monitoring the removal of compounds with different properties and elimination characteristics during various dialysis strategies adds knowledge of dialysis treatment and would be useful for future research in order to decrease complications related to dialysis quality and cardiovascular risk factors. Hopefully the online methods will add a new technique and methodology to the wide discussion about the quality and adequacy of dialysis, uremic toxicity and kidney functionality.
