What is URR in dialysis?

URR (urea reduction ratio) is the percentage by which blood urea nitrogen falls during a single hemodialysis session. It is calculated from a BUN sample drawn just before dialysis and one drawn at the end of the same session.

How do you calculate URR?

Subtract the post-dialysis BUN from the pre-dialysis BUN, divide by the pre-dialysis BUN and multiply by 100. Pre 80 mg/dL and post 24 mg/dL give (80 − 24) ÷ 80 × 100 = 70.0%.

For thrice-weekly hemodialysis, KDOQI sets a minimum URR of 65% and a target of 70%. CMS and the National Kidney Foundation use 65% as the reference for an adequate delivered dose.

How do I convert URR to Kt/V?

Use the Daugirdas second-generation formula: spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W, with R = post ÷ pre BUN, t the session length in hours, UF the fluid removed in litres and W the post-dialysis weight in kg. For URR 70% (R = 0.30), 4 hours, 3 L and 70 kg: −ln(0.268) + 2.95 × 3 ÷ 70 = 1.32 + 0.13 = 1.44.

What Kt/V corresponds to a URR of 65%?

About 1.15 without ultrafiltration in a 4-hour session and about 1.26 with 3 L removed from a 70 kg patient. Because ultrafiltration also removes urea, the same URR gives a higher Kt/V when more fluid is removed, which is why Kt/V is the preferred measure.

What is equilibrated Kt/V?

After dialysis, urea moves back from cells into the blood (rebound), so the dose actually delivered is lower than single-pool Kt/V suggests. eKt/V estimates it: eKt/V = spKt/V − 0.6 × spKt/V ÷ t + 0.03 for a fistula or graft. KDOQI 2015 targets spKt/V 1.4 per session with a minimum of 1.2 for thrice-weekly treatment.

Can I calculate URR with values in mmol/L?

Yes. URR is a ratio, so the unit cancels as long as both values use the same unit. Urea in mmol/L equals BUN in mg/dL × 0.357, and URR is the same whichever is used.

Can this calculator be used for peritoneal dialysis or CKRT?

No. URR and these Kt/V formulas apply to intermittent hemodialysis. Peritoneal dialysis uses weekly Kt/V from dialysate and urine collections, and continuous kidney replacement therapy uses effluent dose.

Is this medical advice?

No. It checks arithmetic only. Dialysis prescription and adequacy decisions belong to the nephrologist and dialysis care team.

Urea Reduction Ratio Calculator

URR = (pre-BUN − post-BUN) ÷ pre-BUN × 100; at least 65% (target 70%) for thrice-weekly hemodialysis. Adding session time, ultrafiltration and post-dialysis weight gives spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W, minimum 1.2, target 1.4 (KDOQI 2015).

URR (urea reduction ratio) is the percentage drop in blood urea nitrogen during one hemodialysis session: URR = (pre-BUN − post-BUN) ÷ pre-BUN × 100.

Paired BUN Inputs

BUN Measurement Unit

Pre-Dialysis BUN (mg/dL)

Post-Dialysis BUN (mg/dL)

Optional: Kt/V (Daugirdas)

Session time (h) Ultrafiltration (L) Post-dialysis weight (kg) Vascular access

Time and weight are needed; blank UF counts as 0 L.

Clearance Assessment

Intermittent HD

Awaiting Paired Lab Values

Enter pre- and post-dialysis blood urea nitrogen values to evaluate the session reduction ratio.

Factual Unit Relationships & Sampling

Dimensionless ratio: The measurement unit cancels from the ratio when both samples use the same analyte and unit: ((k·Pre − k·Post) / (k·Pre)) = (Pre − Post) / Pre. Do not mix mg/dL and mmol/L in one calculation.
Paired treatment requirement: Both blood samples must originate from the same intermittent hemodialysis treatment session.
Sampling protocol fidelity: Use values collected under the dialysis facility's validated pre- and post-treatment sampling protocol. Timing and access recirculation can materially change the result.

Clinical Scope & Kinetic Limitations

No ultrafiltration adjustment: URR evaluates concentration reduction but does not adjust for fluid removed during dialysis; fluid removal changes the relationship between URR and Kt/V.
Urea rebound and generation: URR does not adjust for post-treatment urea rebound from intracellular compartments or ongoing metabolic urea generation during long sessions.
Intermittent hemodialysis only: Do not use this result for peritoneal dialysis or continuous kidney replacement therapy.
Not a prescription directive: Mathematical results below or above 65% do not independently justify altering dialyzer membranes, blood flow rates (Qb), dialysate flow rates (Qd), or treatment duration. Clinical prescription adjustments remain the responsibility of the supervising nephrologist.

URR to single-pool Kt/V

Daugirdas II for a 4-hour session. Ultrafiltration adds Kt/V because removed fluid also carries urea.

URR	R = post ÷ pre	spKt/V, no UF	spKt/V, UF 3 L, 70 kg
60%	0.40	1.00	1.11
65%	0.35	1.15	1.26
70%	0.30	1.32	1.44
75%	0.25	1.52	1.66
80%	0.20	1.78	1.93

Formulas. URR = (Pre − Post) ÷ Pre × 100. spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W, with R = Post ÷ Pre, t in hours, UF in litres and W the post-dialysis weight in kg. eKt/V = spKt/V − 0.6 × spKt/V ÷ t + 0.03 for a fistula or graft, or − 0.47 × spKt/V ÷ t + 0.02 for a venous catheter.

Targets (thrice-weekly hemodialysis). KDOQI 2015: spKt/V target 1.4 per session, minimum delivered 1.2. URR minimum 65%, target 70%.

Worked example. Pre-BUN 80 mg/dL and post-BUN 24 mg/dL give URR 70.0% and R = 0.30. With a 4-hour session, 3 L of ultrafiltration and a 70 kg post-weight: −ln(0.30 − 0.032) = 1.317, plus (4 − 1.05) × 3 ÷ 70 = 0.126, so spKt/V = 1.44, and eKt/V = 1.44 − 0.6 × 1.44 ÷ 4 + 0.03 = 1.26 with a fistula.

Units. Urea (mmol/L) = BUN (mg/dL) × 0.357; URR is identical in either unit as long as both samples use the same one.

References and data handling

CMS: Dialysis Adequacy Quality Measure fact sheet — formula and the 65% URR reference.
National Kidney Foundation: Key points about dialysis — patient-facing 65% reference.
National Kidney Foundation clinical pocket guide — URR formula and kinetic limitations.
KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update — spKt/V target 1.4, minimum 1.2.
Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V. J Am Soc Nephrol 1993;4:1205–1213 — spKt/V formula.
CMS: Current ESRD QIP measures — current program context.

Not medical advice: this tool checks arithmetic for education and lab review. Dialysis dose and prescription decisions belong to the nephrologist and dialysis care team.

Privacy: This calculation runs in your browser. The entered BUN values are not sent to a URR calculation endpoint or stored by this tool.

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Quick answer

URR (urea reduction ratio) is the percentage drop in blood urea nitrogen during one hemodialysis session: URR = (pre-BUN − post-BUN) ÷ pre-BUN × 100. KDOQI sets a minimum of 65% and a target of 70% for thrice-weekly hemodialysis; the matching single-pool Kt/V minimum is 1.2 and the target 1.4.

Example: Pre-BUN 80 mg/dL, post-BUN 24 mg/dL: (80 − 24) ÷ 80 × 100 = 70.0%. With a 4-hour session, 3 L ultrafiltration and 70 kg post-weight, spKt/V = 1.44

Enter paired pre- and post-BUN, add time, UF and weight for Kt/V, then copy the summary.

Summary

How it works

Enter pre-dialysis BUN drawn immediately before the session and post-dialysis BUN drawn at the end of the same session, in the same unit.
The tool computes URR (%) = (Pre − Post) ÷ Pre × 100 and compares it with the 65% minimum and 70% target.
Optionally add session length (hours), ultrafiltration volume (L) and post-dialysis weight (kg) to get single-pool Kt/V with the Daugirdas second-generation formula: spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W, where R = Post ÷ Pre.
Equilibrated Kt/V adjusts for urea rebound with the Daugirdas rate equation: eKt/V = spKt/V − 0.6 × spKt/V ÷ t + 0.03 for an arteriovenous access (− 0.47 and + 0.02 for a venous catheter).
The tool also shows the post-dialysis BUN that would give a URR of exactly 65% and 70% for the entered pre-BUN.

Use cases

Dialysis nurses and clinicians checking session URR against the 65% reference.
Converting a URR into single-pool and equilibrated Kt/V when session time, ultrafiltration and weight are known.
Explaining why the same URR gives a higher Kt/V when more fluid is removed.
Medical, nursing and dialysis-technician students learning urea kinetic formulas.
Patients reading their monthly lab report to discuss dialysis dose with their nephrologist.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu