URR Calculator (urea reduction ratio for hemodialysis)

Understanding the URR calculator — concept and principle

The urea reduction ratio (URR) is the share of blood urea removed during one haemodialysis session. It is one of the simplest measures of whether a patient received an adequate dialysis dose.

Urea removed ÷ before: with the default inputs the result is 68.6% (meets the 65% minimum).

How it is calculated

URR (%) = (pre-dialysis urea − post-dialysis urea) ÷ pre-dialysis urea × 100

Urea removed = pre − post (mg/dL if entered as BUN in mg/dL)

Any urea or BUN unit works if both samples use the same one.
Minimum for thrice-weekly dialysis: 65%.

The principle

Urea is a small waste molecule produced continuously from protein breakdown. Dialysis removes it from the blood, so the fall in concentration over a session shows how much clearance occurred. A URR of 65% corresponds roughly to a single-pool Kt/V of 1.2. URR is easy to calculate, but it ignores urea removed with fluid and urea produced during the session.

Worked example

Default inputs: pre-dialysis BUN 70, post-dialysis BUN 22 mg/dL.

Urea removed = 70 − 22 = 48 mg/dL.
URR = 48 ÷ 70 × 100 = 68.6%.
68.6 ≥ 65 → Yes, meets the minimum.

Each point is the calculator re-run with only “Post-dialysis BUN or urea” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Post-dialysis BUN or urea	Urea reduction ratio (URR)
5.5	92.1%
11	84.3%
16.5	76.4%
22	68.6%
27.5	60.7%
33	52.9%
44	37.1%

How to read the result

Where does your result fall?

For thrice-weekly haemodialysis, a URR of at least 65% has long been the accepted minimum, and many units aim for 70% or more. Kt/V gives a fuller picture because it accounts for ultrafiltration and urea generation. A low URR prompts a check of session length, blood flow, access recirculation and sampling technique.

Common mistakes

Drawing the post-dialysis sample without slowing the blood pump, which dilutes it with recirculated blood.
Drawing it too late, after urea rebounds from tissues.
Comparing URR across very different treatment schedules.

Keep in mind: This is a calculation aid, not medical advice. Dialysis prescriptions are set by the nephrology team.

Frequently asked questions

What is a good URR?

At least 65% per session; many units aim for 70% or more.

What is the difference between URR and Kt/V?

URR uses only the urea fall; Kt/V also accounts for fluid removal and urea generation.

Why does urea rebound after dialysis?

Urea moves back into the blood from tissues over 30–60 minutes.

Sources: National Kidney Foundation, KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update, Am J Kidney Dis 2015;66(5):884–930 · Owen WF Jr, Lew NL, Liu Y, Lowrie EG, Lazarus JM, "The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis", N Engl J Med 1993;329(14):1001–1006

Checked 9 Nov 2026. URR = 1 − post-dialysis BUN ÷ pre-dialysis BUN as defined by the Yale Hemodialysis Kinetics resource; the 65% minimum for thrice-weekly hemodialysis is a standard value from the KDOQI hemodialysis adequacy guidelines.

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