Introduction
The URR (Urea Reduction Ratio) calculator measures how well a dialysis session cleans waste from your blood. During dialysis, a machine filters out urea, a waste product your kidneys can no longer remove on their own. By comparing your blood urea nitrogen (BUN) level before and after treatment, this tool shows the percentage of urea that was removed.
A higher URR means more waste was cleared. The National Kidney Foundation's 2006 KDOQI guideline set a URR of 65% as the minimum dose for sessions under five hours, with 70% as the target.1 A result below 60% has been linked to a higher risk of death and may mean changes to your treatment are needed.2
To use this calculator, enter your pre-dialysis and post-dialysis BUN values. You can choose from several units, including mg/dL and mmol/L. The tool will calculate your URR, show a step-by-step solution, and tell you whether your result meets the recommended target.
How to Use Our URR Calculator
Enter your pre- and post-dialysis blood urea nitrogen (BUN) values below. The calculator will show your urea reduction ratio (URR) as a percentage, tell you if your dialysis session was adequate, and give you a step-by-step breakdown of the math.
Pre-Dialysis BUN / Urea Level: Type in the BUN or urea number from the blood sample taken right before your dialysis session. Then pick the unit your lab used from the dropdown menu (mg/dL, mmol/L, mg/L, g/L, g/dL, or mg%).
Post-Dialysis BUN / Urea Level: Type in the BUN or urea number from the blood sample taken right after your dialysis session. Then pick the matching unit from the dropdown menu. This value should be lower than the pre-dialysis value.
Once both values are entered, press the Calculate button to see your URR result. Press Reset at any time to clear your entries and start over.
What Is the Urea Reduction Ratio (URR)?
The urea reduction ratio, or URR, is a simple way to measure how well a dialysis session cleans your blood. It compares the level of urea (a waste product) in your blood before and after dialysis. The result is shown as a percentage. A higher percentage means more waste was removed during the session.
How Is URR Calculated?
The URR formula is straightforward. You subtract the post-dialysis urea level from the pre-dialysis urea level, divide by the pre-dialysis level, and multiply by 100. In simple terms:
URR = ((Pre-dialysis BUN − Post-dialysis BUN) ÷ Pre-dialysis BUN) × 100
BUN stands for blood urea nitrogen. It is a common blood test that shows how much urea waste is in your blood. Your doctor orders this test before and after each dialysis session to check how well the treatment is working. Doctors often evaluate the BUN result alongside creatinine levels. This formula is essentially a percent change calculation applied to your urea levels.
What Is a Good URR?
The National Kidney Foundation's 2006 KDOQI guideline set 65% as the minimum URR for sessions shorter than five hours, with 70% as the target.1 In a study of 13,473 dialysis patients, those with a URR below 60% had a higher risk of death than patients at 65% to 69%.2 The 2015 KDOQI update measures the dose as spKt/V instead — target 1.4 per session, minimum 1.2 — and says URR should be phased out in favour of more precise methods.3 A result between 60% and 65% is borderline and may need a closer look from your care team.
Why Does URR Matter?
When your kidneys stop working properly, waste builds up in your blood. Dialysis acts like an artificial kidney and filters out that waste. If dialysis does not remove enough urea, the leftover toxins can cause serious health problems over time, including poor nutrition, nerve damage, and heart issues. Tracking your URR helps your doctor make sure your treatments are strong enough to keep you safe. Alongside URR, your care team will typically monitor your glomerular filtration rate (GFR) and creatinine clearance to get a complete picture of your kidney function. Monitoring urine output can also provide useful information about residual kidney function in dialysis patients.
What Can Cause a Low URR?
Several things can lead to a URR that is too low:
- Short treatment time: ending a dialysis session too early means less waste gets removed.
- Low blood flow rate: if blood moves through the dialysis machine too slowly, it cannot clean as much.
- Access problems: issues with the catheter or fistula used for dialysis can reduce how well the machine works.
- Recirculation: this happens when cleaned blood loops back into the machine instead of returning to the body, making the session less effective.
URR vs. Kt/V
URR and Kt/V are both used to measure dialysis adequacy. URR is easier to calculate because it only needs two blood test numbers. Kt/V is more detailed and also accounts for fluid removal and body size. Many clinics use both measures together to get a full picture of how well dialysis is working.
Important Notes
This calculator is an educational tool. It does not replace medical advice. Always share your results with your nephrologist (kidney doctor) so they can make the best decisions about your care. Blood samples must be drawn at the correct times, right before and right after dialysis, to get an accurate URR. Kidney patients should also work with their care team to manage nutrition, since dietary management is a key part of kidney disease care.