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URR Calculator - Urea Reduction Ratio

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Steven Wooding
Based on research by
Owen, W. F., Jr, Lew, N. L., Liu, Y., Lowrie, E. G., Lazarus, J. M. The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis; The New England journal of medicine; September 1993
Last updated: Jan 18, 2024

The URR calculator allows you to check whether the performed dialysis was of good quality - adequate to the patient's state - using the urea reduction ratio (URR) calculations.

Follow the short text below to remind yourself about the most important issues connected to the URR lab tests, the use of URR in dialysis, and its basic formula.

We'll also refresh your knowledge on the hemodialysis adequacy guidelines. 🚰

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace professional medical advice.

What does URR stand for, and how to use it?

The URR is an abbreviation for the urea reduction ratio - we can also call it the urea reduction percentage (PRU). You should measure it every month or on every 12th dialysis session.

URR is a simple method for assessing the quality of dialysis (a life-saving treatment used in the final stage of kidney failure). It is directly connected to the patient's outcome. The URR assesses the ability of the dialysis to eliminate the waste products from the blood. Because of its simplicity, this ratio can also be used for various comparisons, within one patient's results or the entire nation's dialysis standards.

URR is also closely related to the Kt/V equation- both of these values can be used as dialysis adequacy tests (you can read more about Kt/V in the paragraph below, or in a separate Kt/V dialysis calculator).

❗ The dialysis is considered to be adequate, only if the URR ratio is greater than or equal to 65% (≥65%).

The studies have confirmed that patients with lower than adequate URR values had more health problems and a greater risk of death.

What can we do to increase the URR?

  • Increase time on dialysis;
  • Increase blood flow through the dialyzer - it's the case of both the dialyzer and the quality of the patient's blood vessels;
  • Find and eliminate any circulation problems.

Interested in the topic? Omni has plenty of useful tools to offer. For instance, you can use GFR calculator for detecting a kidney disease at its early stage and urine output calculator for computing fluid balance and urine output rate (ml/kg) per hour.

How to calculate URR in dialysis?

The URR calculations for dialysis are straightforward, and you can easily perform them single-handed! In this section, we'd like to present two different versions of the urea reduction ratio formula - choose the one that suits you best.

1. URR=[(UpreUpost)/Upre]100%2. URR=[1(Upost/Upre)]100%\footnotesize \begin{align*} &1.\ \text{URR} = [(\text{U}_\text{pre}- \text{U}_\text{post})/\text{U}_\text{pre}] \cdot 100\%\\ \\ &2.\ \text{URR} = [1 - (\text{U}_\text{post}/\text{U}_\text{pre})] \cdot 100\%\\ \end{align*}

💡 The choice of urea units is wide (mg/dL, mg%, mg/L, g/L, g/dL); you only need to remember to use the same unit for both urea values.

For example, if a patient's urea level before the dialysis is equal to 40 mg/dL, and afterward is lower than 20 mg/dL, we can perform the following URR calculations:

URR = [(40 mg/dL - 20 mg/dL)/40 mg/dL] × 100%

URR = [20/40] × 100%

URR = ½ × 100% = 50%

Our URR is lower than 65%, which means that the performed dialysis was not adequate.

Still hungry for knowledge? 🍰=📚
Check one of our brilliant, topic-related tools, such as FEUrea for Acute Kidney Injury (AKI) and BUN/ creatinine ratio for prerenal causes of AKI.

URR versus Kt/V

URR can also be calculated using the Kt/V equation for dialysis; we can put the relationship between these two values into the following equation:

(K × t)/V = -ln(1 - URR)


  • K means the dialyzer clearance; describes the blood flow rate in milliliters per minute (mL/min);
  • t is the time of dialysis in minutes;
  • V stands for total body fluid volume;
  • URR is the short version of Urea Reduction Ratio, given in %; and
  • ln is used in mathematics to indicate the use of natural logarithm.


How do I know if my URR value is normal?

The URR value is normal if it is greater than or equal to 65%. If you want to know if your URR value is normal, you will need to calculate it, using:

  • The pre-dialysis urea level and the post-dialysis urea level; or
  • The Kt/V value.

What does a URR value of 55% mean in hemodialysis therapy?

A URR value of 55% in hemodialysis therapy may indicate that the treatment may not reach the desired effectiveness level. The dialysis is considered adequate only if the URR ratio is greater than or equal to 65% (≥65%).

How do I calculate URR from Kt/V?

To calculate the URR from the value of Kt/V, you can use the following equation:
(K × t)/V = -ln(1 - URR)


  • K is the dialyzer clearance;
  • t is the time of dialysis in minutes; and
  • V is the total body fluid volume.

Of course, it is easier if you use Omni's URR Calculator.

What is the pre-dialysis urea level if the URR is 70%?

50 mg/dL. To calculate it, you will need to know the following:

  • The post-dialysis urea level. Let’s assume that the post-dialysis is 15mg/dL.
  • The URR. In our case, we will assume it is 70%.
  • That’s it. If you apply these values in the equation URR=[(Upre−Upost)/Upre]⋅100%, you will find that the pre-dialysis urea level is 50 mg/dL. You can verify it using our calculator!
Łucja Zaborowska, MD, PhD candidate
Pre-dialysis urea level
Post dialysis urea level
Urea reduction ratio (URR)
💡 If you're looking for the Kt/V vs. URR relationship, try the Kt/V calculator.
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