Urea Reduction Ratio (URR) Calculator
Urea Reduction Ratio (URR) Calculator
Calculate the urea reduction ratio, the simplest bedside measure of dialysis adequacy, from pre- and post-dialysis BUN.
Urea Reduction Ratio (URR)
Pre + post BUNPre-dialysis BUN 60 mg/dL, post-dialysis BUN 18 mg/dL
Formula
- pre-dialysis BUN
- mg/dL, drawn immediately before the session starts
- post-dialysis BUN
- mg/dL, drawn at the end of the session using a technique that avoids recirculation
Worked example
Pre-dialysis BUN 60 mg/dL, post-dialysis BUN 18 mg/dL
1 − (18 ÷ 60) = 1 − 0.300 = 0.700
0.700 × 100 = 70.0%
Between 65% and 80% → adequate
Adequacy thresholds
| URR | Adequacy | Approximate Kt/V |
|---|---|---|
| < 65% | Below target | < 1.2 |
| 65 – 80% | Adequate | ≈ 1.2 – 1.8 |
| > 80% | High | > 1.8 |
A simple ratio with real limitations
The urea reduction ratio was in wide use before Kt/V became standard, and it survives because it needs only two numbers and no logarithm — a fraction fall in urea, expressed as a percentage. That simplicity is also its weakness: URR takes no account of treatment time, blood and dialysate flow rates, or ultrafiltration, all of which affect how much urea is actually removed for a given reduction in concentration.
Ultrafiltration matters more than it first appears. Fluid removal during dialysis dilutes urea distribution volume, so a large ultrafiltration volume can produce a URR that looks adequate even when convective clearance and overall dose are borderline. Kt/V corrects for this explicitly; URR does not, which is why guidelines now specify Kt/V as the primary target and quote URR as a secondary, easier-to-calculate check.
The rough correspondence often quoted — a URR of 65% equating to a Kt/V of about 1.2 — holds only approximately and breaks down at higher ultrafiltration volumes or unusual session lengths. The same sampling pitfalls that distort Kt/V apply here too: a post-dialysis sample drawn before urea rebound, or contaminated by access recirculation, understates true post-dialysis urea and inflates the calculated URR. URR remains useful as a quick adequacy screen and for trending a patient over time, but a single measurement should not be used to fine-tune a prescription in isolation from Kt/V.
Frequently asked questions
What is a good urea reduction ratio?
65% or above is generally considered adequate for thrice-weekly haemodialysis, corresponding roughly to a Kt/V of 1.2. A target above 65–70% is more typical in practice.
Why is Kt/V preferred over URR?
URR ignores ultrafiltration volume and residual renal function, both of which affect the true dialysis dose. Kt/V corrects for these directly, which is why it is the primary adequacy target in current guidelines.
Does URR use the same blood samples as Kt/V?
Yes — the same pre- and post-dialysis BUN values are used for both. The sampling technique matters equally for either calculation.
Can URR and Kt/V disagree?
Yes, particularly at high ultrafiltration volumes, where URR can look adequate while Kt/V, which accounts for fluid removal, is borderline or low.
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References
- Owen WF Jr et al. The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis. N Engl J Med. 1993;329(14):1001–6.
- National Kidney Foundation. KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update. Am J Kidney Dis. 2015;66(5):884–930.