Urea reduction ratio
The urea reduction ratio (URR) is a dimensionless number used to quantify dialysis treatment adequacy. Definition. formula_1 Where: Whereas the URR is formally defined as the urea reduction "ratio", in practice it is informally multiplied by 100% as shown in the formula above, and expressed as a percent. History and overview. The URR was first popularized by Lowrie and Lew in 1991 as a method of measuring amount of dialysis that correlated with patient outcome. This method is very useful because of its simplicity. It permits easy monitoring of the amount of dialysis therapy delivered to individual  patients, as well as across dialysis units, groups of units, states, regions, or countries, because monthly predialysis and postdialysis urea nitrogen values are routinely measured. It also permits quality control and improvement initiatives and regulatory oversight. The United States Renal Data Systems (USRDS) publishes annual data regarding the URR values being delivered to dialysis patients across the United States. The ESRD networks monitor therapy across groups of states. The European Renal Association (ERA-EDTA) Registry covers most European countries, and DOPPS (Dialysis Outcomes Practice Patterns Study) records and analyzes URR and other data from selected dialysis units located in countries  across the world. Relation to Kt/V. Mathematically, the URR is closely related to Kt/V, and the two quantities can be derived from another with more or less precision, depending on the amount of additional information available about a given dialysis session. Kt/V is one of the reference methods by which the amount of dialysis given is measured. Kt/V, like the URR, focuses on urea as the target solute, and is based on the assumption that removal of urea is from a single space – "urea distribution volume", or formula_2 similar in capacity to the total body water . The urea distribution volume formula_2, although traditionally thought of as 60% of body weight, may actually be closer to 50% of the body weight in women and 55% in men with stage V (GFR < 15 ml/min) chronic kidney disease. The clearance of urea during the dialysis session formula_4 can be expressed in either formula_5 or formula_6. Time or formula_7 is the duration of the dialysis session, measured either in minutes or hours. So formula_8 is also a volume, either formula_9, or formula_10, and represents the volume of blood (in ml or L) cleared of urea during the dialysis  session. Because formula_2 is also a volume, the ratio of formula_12 has dimensions of formula_13 or formula_14, making it a "dimensionless" ratio. In a simplified model of urea removal from a fixed volume with no urea generation, formula_15 is related to formula_16 by the following relationship: formula_17 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. formula_18 The formula_19 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 , formula_20 adjusts for the additional urea that is cleared from the body through volume contraction. Because formula_21 can be approximated by formula_22, 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: formula_23 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 zero. In long slow overnight dialysis, if simplified equations are used, the URR also underestimates the amount of dialysis due to urea generation during the long dialytic session. For this reason, the kinetically modeled Kt/V is always recommended as the best measure of dialysis adequacy. The Kt/V, even that derived by formal modeling, is primarily based on the URR, and so it contains little additional information in terms of the amount of dialysis that was delivered . Since the "URR" and "Kt/V" are so closely related, their predictive power in terms of patient outcome is similar. However, use of "Kt/V" and urea modeling in general allows for comparing expected with predicted dose of dialysis, which can be used to analyze dialysis treatments and dialyzer clearances and in troubleshooting and quality control activities. Also, Kt/V permits calculation of the urea generation rate, which can give clues about a patient's protein intake. Minimally adequate dose in terms of URR. In the standard 3x/week hemodialysis schedule a "URR" of 65% is considered the minimum acceptable  dose, corresponding to a minimum "Kt/V" of 1.2 When dialysis is given more frequently than three times a week, the minimum acceptable URR is lower; because more dialysis treatments are given over the week, the dose of dialysis for each treatment does not need to be as large. Also minimally acceptable values for URR (and Kt/V) can be reduced in patients who have substantial amounts of residual renal function.