**2. Dialysis adequacy and online monitoring**

Dialysis adequacy means providing a sufficient amount of dialysis treatment to maintain a uremic patient in the best condition. The goal of qualitatively treated dialysis is to prevent complications due to uremic toxicity.

Some recent studies are reviewed suggesting that uremic toxins are involved in the progression of renal failure and are at least partially removed by hemodialysis (Lesaffer, 2000; Eloot, 2008; Vanholder, 2003). The efficiency of a dialysis session has been estimated through concentration of uremic toxins measured before and after the dialysis session. A classic marker of the dialysis dose and adequacy is a small molecule urea.

Two methods are generally used to assess dialysis adequacy: URR and Kt/V. The urea reduction ratio (URR) is based on tests of blood urea, by measuring the levels before and after treatment to show how much has been removed. The Kt/V is mathematically related to the URR and can be derived from it (NKF-DOQI, 2006). In a simplified model of urea removal from a fixed volume with no urea generation, Kt/V is related to URR as follows:

$$\text{Kt/V} = \text{-In} \,(1 - \text{URR}) \tag{1}$$

In general, this method of dialysis adequacy is based on pre- and post-dialysis measurements of urea concentration. Online methods are considered to be more accurate than methods based on pre- and post-dialysis urea concentrations and have been found to be better suited to clinical routines. Online monitoring of the dialysis dose has been suggested as a valuable tool in ensuring adequate dialysis prescription (Locatelli, 2005).
