**4. Monitoring uremic toxins beyond urea**

The fact that urea is considered generally a nontoxic substance and only a marker for uremic retention solutes, and the EUTox has identified several more relevant uremic toxins utilizing optical analysis methods, arise a question connected to further development of optical techniques "Could some of these identified uremic toxins be measured optically on-line?" Of the 90 compounds that have been identified as uremic toxins by the EUTox group [51, 52, 70], we have, from spectroscopic databases, identified 36 to be UV absorbing and among them approximately 25 to be absorbing near 297 nm. In addition to these 90 compounds mentioned as uremic toxins, there are even more solutes in the dialysate that are optically active at 297 nm, which add to the measured UV absorbance signal. Spent dialysate contains numerous different absorbing compounds and concentrations of solutes decline differently during a dialysis session for individuals. The UV absorbance curve may therefore be an individual "clinical print" of the patient's sum of several UV absorbing solutes and therefore a possible parameter for monitoring total solute removal during dialysis. Analyses still remain to find the single solute's individual contribution to the absorbance signal, which is also dependent on which wavelengths are used [71]. Earlier knowledge from the correlation analysis between UV absorbance and a few uremic toxins has shown that it is possible to estimate removal of such solutes, which have a high correlation to UV absorbance. This removal of other uremic retention solutes beyond urea may have stronger impact on dialysis outcomes compared to urea or urea alone.
