is also a volume, the ratio of has dimensions of or , making it a "dimensionless" ratio. In a simplified model of urea removal from a fixed volume with no urea generation, is related to by the following relationship: Actually, this relationship is made a bit more complex by the fact that fluid is removed during dialysis, so the removal space V shrinks, and because a small amount of urea is generated during the dialysis session. Both of these factors make the actual post-dialysis serum urea level higher than expected, and the URR lower than expected, when the extremely simplified equation above, is used. A more accurate relationship between URR and Kt/V can be derived by single-pool, variable volume urea kinetic modeling. A simplified estimating equation also can be used. This gives results that are quite similar to formal urea modeling as long as dialysis treatments of 2–6 hours in duration are given, and Kt/V is between 0.7 and 2.0. The term is a function of the dialysis session duration (t), and adjusts for the amount of urea generated during the dialysis session. The second term, adjusts for the additional urea that is cleared from the body through volume contraction. Because can be approximated by , where UF = ultrafiltrate removed during dialysis (estimated as the weight lost during the treatment) and W = postdialysis body weight, and because dialysis sessions given 3 times per week are usually about 3.5 hours long, the above equation can be simplified to: Nomogram relating Kt/V and URR Instead of equations, a nomogram can be used to easily estimate Kt/V from the URR in clinical practice. To use the nomogram, one needs to know the postdialysis weight (W) as well as the amount of weight (fluid) loss during the dialysis session (UF). First, find the URR on the vertical axis, then move over to the proper isopleth (curved line) depending on the amount of weight lost during dialysis (UF/W). Then drop down to the horizontal axis to read off the Kt/V value: Limitations of URR vs. Kt/V The URR is designed to measure the amount of dialysis given when the dialysis clearance of urea greatly exceeds the urea generation rate. In continuous hemodialysis or in peritoneal dialysis, for example, a considerable amount of dialysis is delivered, but the urea level remains roughly constant after the initial treatment of uremia, so the URR is essentially