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.

URR Calculator - Urea Reduction Ratio

By Łucja Zaborowska
Last updated: Dec 22, 2020

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. 🚰

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 nations' 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:

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 = [(Upre - Upost)/Upre] * 100%

  2. URR = [1 - (Upost/Upre)] * 100%


  • URR is the Urea Reduction Rate, given in %;
  • Upre stands for the urea level before the dialysis; and
  • Upost stands for the urea level after the dialysis.
💡 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:

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.
Łucja Zaborowska