URR (Urea Reduction Ratio) Calculator & Kt/V Estimate
Calculate the percentage drop in blood urea (BUN) during hemodialysis, compare it with the KDOQI ≥ 65% target, and get a spKt/V estimate — complete with Daugirdas formula mode.
Medical disclaimer: This calculator is an educational tool for understanding what URR numbers mean. Its results are not a diagnosis and not a recommendation to change dialysis dose. All treatment decisions are made together with the doctor/nephrologist caring for you.
Terminology note: “URR” here is the urea reduction ratio for assessing hemodialysis adequacy — not a login portal. Different from the BUN/creatinine ratio calculator, which compares BUN against creatinine to assess the cause of elevated BUN.
2. Advanced mode: Daugirdas formula
The Daugirdas single-pool Kt/V formula includes session length (t), ultrafiltration (UF), and body weight (W):
spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × (UF / W), where R = post-dialysis BUN / pre-dialysis BUN.
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Educational estimate based on the Daugirdas single-pool formula. Still requires clinical interpretation by a nephrologist.
6. Frequently asked questions (FAQ)
Medical disclaimer: answers are educational information, not individual medical advice.
What is URR (Urea Reduction Ratio)?
URR is the percentage drop in blood urea (BUN) during one hemodialysis session. The formula: URR = (pre-dialysis BUN minus post-dialysis BUN) divided by pre-dialysis BUN, times 100 percent. Example: pre-dialysis BUN 80 mg/dL and post-dialysis 24 mg/dL gives a URR of 70.0 percent. URR is used as a simple indicator of dialysis dose adequacy.
What URR value is considered adequate?
Per KDOQI guidelines, a minimum URR of 65 percent is the threshold for adequate dialysis in routine thrice-weekly hemodialysis patients. Values below 65 percent signal that the dialysis dose needs evaluation with the nephrology team.
What is the difference between URR and Kt/V?
Both measure dialysis adequacy, but Kt/V accounts for the body’s urea distribution volume and includes ultrafiltration and session-time corrections, while URR only compares pre- and post-dialysis BUN. Single-pool Kt/V with a KDOQI minimum target of 1.4 is the more complete measure.
How do I estimate Kt/V from URR?
Simple estimate: spKt/V equals the negative natural logarithm of (1 minus URR/100). For a URR of 70 percent, the result is about 1.20. This is only an approximation because it ignores ultrafiltration and urea rebound. A more accurate estimate uses the Daugirdas formula, which includes session length, ultrafiltration, and body weight.
When can URR results be misleading?
URR can look fine even when the dose is inadequate if: the post-dialysis BUN sample is drawn too late so urea rebound occurs; ultrafiltration is very large and uncorrected by URR; the vascular access is faulty with recirculation; or the session is very short with large post-dialysis rebound.
How should BUN samples be drawn correctly?
The pre-dialysis sample is drawn before the session starts. The post-dialysis sample is drawn immediately after dialysis ends, ideally with a technique that minimizes rebound effects per the dialysis unit’s protocol. Late sampling makes URR look lower than its true value.
What factors affect URR?
URR rises with longer dialysis duration, higher blood flow rate (Qb), a large-capacity dialyzer (high KoA), and well-functioning vascular access. URR falls when sessions are shortened, Qb is low, the dialyzer is small or partially clotted, or access recirculation occurs.
What BUN units does this calculator use?
The calculator accepts mg/dL and mmol/L, where 1 mg/dL equals 0.357 mmol/L. Because URR is a ratio, the result is the same whatever the units, as long as pre- and post-dialysis values use the same unit.
What is spKt/V and what is its target?
spKt/V (single-pool Kt/V) is a dialysis dose measure that accounts for dialyzer clearance, time, and urea distribution volume. KDOQI guidelines set a minimum spKt/V target of 1.4 per session for thrice-weekly hemodialysis.
Why is post-dialysis BUN higher than pre-dialysis?
That is unusual and usually indicates a sampling or labeling error, or a post-dialysis sample drawn after extreme urea rebound. Discuss with the dialysis team so the measurement can be repeated with proper technique.
Does a URR of 70 percent mean my dialysis is adequate?
A URR of 70 percent meets the KDOQI minimum target of 65 percent, but dialysis adequacy is assessed comprehensively: clinical symptoms, nutritional status, blood pressure, anemia, and other laboratory results. This calculator’s figures are educational and do not replace a nephrologist’s assessment.
Does this calculator replace the doctor’s decisions?
No. This calculator is an educational tool for understanding what URR numbers and Kt/V estimates mean. All decisions about dialysis dose must be made together with the doctor and nephrology team caring for you.