**3. Dialysis monitoring utilizing UV absorbance technique**

Several solutes in spent dialysate identified as uremic toxins by Prof. Vanholder and colleagues in the European Uremic Toxin Work Group (EUTox) have been measured by applying UV absorbance in HPLC technique [49–52]. Currently, two optical techniques for dialysis dose monitoring have been investigated—UV absorbance and NIRS [53, 54]. Other approaches have also been investigated but are more limited so far, for example utilizing the Vis region for measurement. In this chapter,

the UV absorbance technique will be discussed that is applied in the commercialized dialysis adequacy monitors ADIMEA [55] and DDM [56]. We will present how clinical parameters based on urea, such as Kt/V, urea reduction ratio (URR), total removed urea (TRU) and protein catabolic rate/protein nitrogen appearance (PCR/PNA), can be estimated optically utilizing UV absorbance.

### **3.1 Present clinical parameters from optical dialysis dose monitoring**

Urea kinetic modeling (UKM), that is where urea is used in differential equations, with the attempt to provide quantitative assessments of dialysis and nutrition adequacy in dialysis patients. The high correlation between urea concentration and UV absorbance values gives consequently the possibility to utilize the UKM equations for UV absorbance similarly. During the UV absorbance and on-line measurements, the pure dialysate was used as the reference, (Eq. (3)) Ir = pure dialysate, and the wavelength was fixed for the entire dialysis. The absorbance baseline level was after the pure flowing dialysate had been stabilized in temperature and conductivity, set to zero when the pure dialysate was flowing through the cuvette prior treatment, see **Figure 3** for the schematic set-up.
