URR is the simplest measure of dialysis dose. It does not account for fluid removal or urea generation during dialysis, so Kt/V is more precise, but URR is widely used for monthly quality monitoring.
How to use the URR Calculator
- Pre-dialysis BUN
- Post-dialysis BUN
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter BUN (or urea) before and after the dialysis session.
Inputs explained
| Field | What to enter |
|---|---|
| Pre-dialysis BUN | Units: mg/dL, mmol/L urea |
| Post-dialysis BUN | Units: mg/dL, mmol/L urea |
What each result means
- Urea reduction ratio = ([Pre-dialysis BUN] – [Post-dialysis BUN])÷[Pre-dialysis BUN]×100
- Approximate Kt/V (no ultrafiltration) = -ln([Post-dialysis BUN]÷[Pre-dialysis BUN])
Example values
How the main result changes when Pre-dialysis BUN changes and everything else stays at the example values above:
| Pre-dialysis BUN (mg/dL) | Urea reduction ratio (%) |
|---|---|
| 30 | 33 |
| 45 | 56 |
| 60 | 67 |
| 75 | 73 |
| 90 | 78 |
| 120 | 83 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
URR (%) = (pre-BUN − post-BUN) ÷ pre-BUN × 100
Worked example
Inputs — Pre-dialysis BUN: 60 mg/dL; Post-dialysis BUN: 20 mg/dL.
Results — Urea reduction ratio = 67 %; Approximate Kt/V (no ultrafiltration) = 1.1.
Key terms
- URR
- Urea reduction ratio.
Frequently asked questions
What is a good URR?
At least 65%, ideally 70% or more.
How often is URR measured?
Usually once a month.
Why might URR be low?
Short sessions, low blood flow, access recirculation or a clotted filter.