**1. Introduction**

The traditional surrogate marker for dialysis dose, urea, which should reflect the clearance of various toxins and metabolic end products is disputed [1]. Removing a sufficient quantity of urea makes it possible to reduce symptoms, morbidity and mortality, and improve quality of life [2]. Urea has over the years been attempted to be measured online in the spent dialysate with different techniques such as enzymatic-, conductivity- and optical sensors. The enzymatic technique measure urea concentrations in the effluent spent dialysate stream, online, using either an ammonium ion (NH4 +) sensor that measures the amount of NH4 + determined directly by an ion-specific electrode or by an electrical potential difference between two electrodes, generated from hydrolysis of urea produces NH4+. A urease membrane catalyzes the chemical reaction when it comes in contact with urea in dialysate [3, 4]. This online technique has disappeared from the market possibly due to cumbersome handling for the staff and high extra costs. There are two still present techniques, first the ionic dialysance method uses a conductivity sensor [5] and is based on the fact that the diffusion coefficients of sodium and urea are similar at 37°C, through a dialysis membrane, therefore sodium dialysance can be used as a marker for urea clearance. The second commercially available technique is the optical method, which will be presented in more detail in this chapter. This technique utilizes the high correlation, in spent dialysate, between urea and UV absorbance at a certain wavelength range, even when urea itself does not absorb UV light [6, 7].

Urea shows a kinetic behavior that is not representative of all retained uremic toxins, including other water-soluble molecules belonging to the group of small molecules. A more comprehensive picture is needed for assessment of uremic solute removal during dialysis involving kinetic profiling and monitoring of the key molecules of all three groups (small-, middle- and protein-bound molecules) of solutes in uremic toxicity [1, 8, 9]. European Renal Best Practice has pointed out that beta-2 microglobulin (β2M) is a potential marker for the middle-size group having a kinetic behavior sufficiently representative of other middle molecules, including peptides of similar size [10, 11]. The protein-bound group, indoxyl sulphate (IS), has received attention because of its link to cardiovascular disease and mortality [12]. Furthermore, analyzing concentrations of these molecules requires today the cumbersome highperformance liquid chromatography (HPLC) method, which, therefore, has limited possibilities to be used in daily clinical practice.

A combined optical online technology utilizing simultaneously both UV absorbance and fluorescence might be a solution for this. This chapter will mainly focus on the latest research and development in that direction and will conclude with the results achieved so far.
