The Urea Reduction Ratio (URR) is a simple numerical index used to assess the adequacy of a hemodialysis session. It expresses how much urea has been removed during one dialysis treatment, based on blood urea levels before and after dialysis.
What is URR and how is it calculated?
URR is defined as the percentage fall in blood urea during a dialysis session: URR=Upre−UpostUpre×100%\text{URR} = \frac{U_{\text{pre}} – U_{\text{post}}}{U_{\text{pre}}} \times 100\%URR=Upre​Upre​−Upost​​×100%
Where:
- Uₚᵣₑ = pre-dialysis blood urea (or BUN)
- Uₚₒₛₜ = post-dialysis blood urea
Because the same units are used before and after dialysis (e.g. mg/dL or mmol/L), the units cancel out and URR is a percentage.
URR is closely related to Kt/V, another standard dialysis adequacy measure; both correlate with patient outcomes and can often be converted from one to the other.
Urea Reduction Ratio (URR) Calculator
Dialysis adequacy measure using pre- and post-dialysis urea
Enter Laboratory Values
Educational tool only. URR should be interpreted by qualified renal staff and must not replace professional medical advice.
How to Interpret URR
URR is the percentage fall in blood urea during a dialysis session. In thrice-weekly hemodialysis, higher URR generally indicates better dialysis adequacy.
| URR (%) | Interpretation |
|---|---|
| < 60% | Inadequate – usually requires review of dialysis dose. |
| 60–64% | Borderline – may need optimisation. |
| 65–69% | Acceptable – minimum recommended for 3×/week HD. |
| ≥ 70% | Good / optimal – often preferred target in many units. |
Thresholds are approximate and based on guideline and cohort data. Always interpret URR alongside Kt/V, clinical status, residual renal function, and local policies.
