What is URR?
The Urea Reduction Ratio (URR) is a measure of hemodialysis adequacy that quantifies the percentage reduction in blood urea nitrogen (BUN) during a single dialysis session. It is one of the simplest and most widely used metrics for assessing how effectively a dialysis treatment removes uremic toxins.
URR was first proposed as a dialysis adequacy measure by Lowrie and Lew in 1991, who demonstrated a strong correlation between URR and patient survival. Higher URR values indicate more effective toxin removal and are associated with better clinical outcomes, including reduced mortality, fewer hospitalizations, and improved quality of life.
While URR is intuitive and easy to calculate, it has limitations compared to more sophisticated measures like Kt/V, as it does not account for ultrafiltration, urea generation during dialysis, or residual renal function.
URR & Kt/V Formulas
Urea Reduction Ratio
Estimated Kt/V (from URR)
A simplified estimation of single-pool Kt/V can be derived from URR using the natural logarithm:
This approximation is based on the first-order kinetics of urea removal and is reasonably accurate for URR values between 50% and 75%. More precise Kt/V calculations (Daugirdas second-generation formula) also account for ultrafiltration and residual renal function.
Unit Conversion
Urea (mmol/L) ÷ 0.357 = BUN (mg/dL)
Interpretation & Targets
| URR Range | Estimated Kt/V | Assessment | Clinical Action |
|---|---|---|---|
| ≥65% | ≥1.05 | Adequate dialysis | Continue current prescription |
| 60–65% | 0.92–1.05 | Borderline | Review prescription, consider adjustments |
| <60% | <0.92 | Inadequate dialysis | Increase dialysis dose (time, blood flow, or frequency) |
Dialysis Adequacy Diagram
Kt/V Explained
Kt/V is a dimensionless number that represents the fractional clearance of urea during dialysis. Each component has a specific meaning:
- K = Dialyzer clearance of urea (mL/min) — depends on dialyzer membrane, blood flow rate, and dialysate flow rate
- t = Dialysis session time (minutes) — longer sessions increase urea removal
- V = Volume of distribution of urea (mL) — approximately equals total body water (roughly 60% of body weight in males, 55% in females)
A Kt/V of 1.0 means that the total volume of blood cleared of urea during the session equals the patient's total body water volume. The KDOQI target of Kt/V ≥1.2 ensures that at least 1.2 times the body water volume is cleared per session.
Single-Pool vs Double-Pool Kt/V
Urea kinetic modeling recognizes that urea distributes in different body compartments that equilibrate at different rates:
- Single-pool Kt/V (spKt/V): Assumes urea is uniformly distributed in one compartment. Calculated from immediate post-dialysis BUN. This is the standard clinical measure and is what the URR-derived estimate approximates.
- Equilibrated Kt/V (eKt/V): Also called double-pool Kt/V. Accounts for the "urea rebound" that occurs 30–60 minutes after dialysis ends, as urea equilibrates from intracellular to extracellular compartments. eKt/V is typically 0.15–0.20 lower than spKt/V.
The Daugirdas second-generation formula provides more accurate spKt/V calculations:
Where R = post/pre BUN ratio, t = session time in hours, UF = ultrafiltration volume (L), and W = post-dialysis weight (kg).
KDOQI Guidelines
The National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (KDOQI) provides evidence-based guidelines for dialysis adequacy:
- Minimum delivered dose: spKt/V ≥1.2 per hemodialysis session (thrice weekly)
- Equivalent URR target: ≥65%
- Measure monthly for all patients on maintenance hemodialysis
- The target should be consistently achieved, not just occasionally met
- If target is not met, evaluate access recirculation, dialyzer performance, blood flow rate, session time, and needle placement
- For patients on twice-weekly dialysis, higher per-session targets are needed
Studies have shown that URR <60% (Kt/V <0.9) is associated with significantly increased mortality, while values above 65% show progressively better outcomes up to a plateau.
Factors Affecting Dialysis Adequacy
| Factor | Effect on URR/Kt/V | How to Optimize |
|---|---|---|
| Blood flow rate (Qb) | Higher Qb increases clearance | Target 300–500 mL/min; ensure adequate vascular access |
| Dialysate flow rate (Qd) | Higher Qd improves diffusion gradient | Standard 500–800 mL/min |
| Session time | Longer sessions increase total clearance | Minimum 4 hours for thrice-weekly HD |
| Dialyzer membrane | Larger surface area increases K | Match dialyzer to patient size |
| Access recirculation | Reduces effective clearance | Monitor recirculation; correct needle placement |
| Patient size (V) | Larger patients need more clearance | Adjust prescription to body size |
Worked Example
A patient has pre-dialysis BUN of 80 mg/dL and post-dialysis BUN of 24 mg/dL:
A URR of 70% exceeds the KDOQI target of 65%, and the estimated Kt/V of 1.20 meets the minimum target of 1.2. This represents adequate dialysis.
Frequently Asked Questions
What is a good URR?
A URR of 65% or higher is considered adequate according to KDOQI guidelines. Most well-dialyzed patients achieve URR values of 65–75%. Values above 75% are excellent and indicate very effective dialysis.
How is URR different from Kt/V?
URR is simpler to calculate (only requires pre and post BUN values) but less comprehensive. Kt/V accounts for additional factors like ultrafiltration volume and residual kidney function. In practice, both are used together, with Kt/V being the primary adequacy measure and URR serving as a useful quick check.
When should blood samples be drawn?
Pre-dialysis BUN should be drawn immediately before the session starts (before any saline priming reaches the patient). Post-dialysis BUN should be drawn using the slow-flow or stop-pump method: reduce blood pump to 50–100 mL/min for 15 seconds, then draw the sample from the arterial port. This minimizes access recirculation artifacts.
What if my URR is below 65%?
Consistently low URR values require investigation. Common causes include: shortened treatment times (patient non-compliance or facility scheduling), inadequate blood flow rates, access dysfunction (stenosis, recirculation), clotted or degraded dialyzer fibers, and incorrect needle placement. Your nephrologist should review the dialysis prescription and vascular access.
Does URR apply to peritoneal dialysis?
No, URR and spKt/V as described here apply specifically to hemodialysis. Peritoneal dialysis adequacy is measured using weekly Kt/V (target ≥1.7) and weekly creatinine clearance, calculated differently from the continuous nature of peritoneal dialysis.