Urea reduction ratio

The urea reduction ratio (URR) is a dimensionless number used to quantify dialysis treatment adequacy.

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.

The URR was first popularized by Lowrie and Lew in 1991 as a method of measuring amount of dialysis that correlated with patient outcome.

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 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.

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.

, and represents the volume of blood (in ml or L) cleared of urea during the dialysis session.

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.

A more accurate relationship between URR and Kt/V can be derived by single-pool, variable volume urea kinetic modeling.

[2] 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.

adjusts for the additional urea that is cleared from the body through volume contraction.

, 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:

Instead of equations, a nomogram can be used to easily estimate Kt/V from the URR in clinical practice.

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: 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.

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.

Also, Kt/V permits calculation of the urea generation rate, which can give clues about a patient's protein intake.

Also minimally acceptable values for URR (and Kt/V) can be reduced in patients who have substantial amounts of residual renal function.