URR Calculator - Urea Reduction Ratio with KDOQI Band

A URR calculator turns a maintenance hemodialysis session's pre- and post-dialysis BUN into a urea reduction ratio percentage, the BUN reduction ratio R, and a KDOQI adequacy band against the published 65% benchmark. The form also shows the UF / W term as a Kt/V reference; the same inputs feed a Kt/V calculator.

• • Single-session review: read one treatment against the KDOQI 65% benchmark and the matching adequacy band.
• • Monthly adequacy average: re-run the form for each session in a month and average the percent drop against the KDOQI monthly target.
• • Home hemodialysis planning: estimate a short daily or alternate-night session from the same BUN inputs.
• • Comparing prescription changes: try a longer session or different ultrafiltration target against the same BUN profile; treat single-session swings as a data point.

Urea reduction ratio is the older single-number benchmark for small-solute removal in maintenance hemodialysis. Many units still put it on the run sheet next to weight and blood pressure.

A session at 65% meets the KDOQI floor, 75% is well above, and 55% sits below. The benchmark is a monthly average, so a single low session is a data point, not an immediate trigger.

When the same pre/post-BUN pair needs the KDOQI 1.2 minimum spelled out, the Kt V Calculator extends the URR percentage into the second-generation spKt/V and the matching KDOQI band.

The form runs the URR percentage, the BUN reduction ratio, the UF / W reference term the Kt/V formula needs, and a KDOQI 65% adequacy band. The URR percentage uses pre- and post-BUN only; ultrafiltration and post-weight do not change the URR number, they only feed the UF / W reference.

• preBUN, postBUN, R: pre- and post-dialysis BUN in mg/dL. R is postBUN / preBUN; an adequate session has R around 0.25 to 0.35 on three-times-weekly in-center hemodialysis.
• ultrafiltration (L): net fluid removed during the session in liters. The UF / W term captures the proportion of body water removed; the URR percentage does not include it, but the Kt/V formula does.
• post-weight (kg): patient weight at the end of the session in kilograms; the denominator of the UF / W reference term.

A URR percentage is the simplest adequacy number a dialysis team can compute from a single pre/post-BUN pair, which is why it has stayed on monthly run sheets since the early National Cooperative Dialysis Study.

R is the post-BUN divided by pre-BUN. When R is 0.25, the URR is 75%; when R is 0.35, the URR is 65%.

The UF / W term is shown alongside the URR percentage for reference. The URR percentage does not change when fluid is removed, because the calculation only uses pre- and post-BUN; the UF / W value is the term the Kt/V formula multiplies by (4 - 3.5 x R) to add the ultrafiltration correction.

According to National Kidney Foundation KDOQI 2020, a delivered URR of 65% or higher supports a minimum spKt/V of 1.2 per session for three-times-weekly in-center hemodialysis, with the 65% cutoff set against the monthly average.

For residual kidney function, the Glomerular Filtration Rate Calculator turns the same serum creatinine, age, and sex inputs into a CKD-EPI eGFR number the team reads alongside the URR.

Four ideas carry most of the clinical meaning behind a URR result.

URR is not a complete picture of dialysis adequacy on its own. KDOQI 2020 lists URR as a supporting benchmark to the minimum spKt/V of 1.2, with a target spKt/V of 1.4 per session.

R near 0.25 to 0.35 is the typical adequate band for a three-times-weekly in-center session. R above 0.40 usually means a short session, an underperforming access, or recirculation to check at the access.

URR captures the small-solute clearance side of the same patient; the FeUrea Calculator reads the same BUN and creatinine pair as a fractional excretion between sessions.

This calculator takes four numbers. Each one should be set to a real session value rather than an idealized one.

1. 1 Enter the pre-dialysis BUN: type the BUN drawn immediately before the session starts, in mg/dL. Most maintenance hemodialysis patients fall between 40 and 80 mg/dL.
2. 2 Enter the post-dialysis BUN: type the post-dialysis BUN from the arterial line using the slow-flow or stop-pump technique, in mg/dL.
3. 3 Enter the ultrafiltration volume: type the net fluid removed during the session in liters, in 0.1-liter steps. Use 0 for an isovolemic session.
4. 4 Enter the post-dialysis weight: type the patient's weight at the end of the session in kilograms.
5. 5 Read the result together: compare the URR percent to the KDOQI 65% benchmark and read the adequacy band label before changing the prescription.

A typical in-center session: preBUN 60, postBUN 15, ultrafiltration 2 L, post-weight 70 kg returns URR 75.0%, R 0.250, UF / W 0.029, and the adequacy band 'exceeds KDOQI 65% benchmark'. If a single session comes back below 65%, note it in the trend; KDOQI 2020 sets the 65% benchmark against the monthly average.

The same pre-session BUN travels with the serum creatinine, so the BUN Creatinine Ratio Calculator reads the BUN-to-creatinine relationship on the same lab draw.

A URR percentage has stayed on the run sheet for decades because it is fast, transparent, and auditable. The URR calculator keeps that one-line format while showing the supporting R ratio and the UF / W reference term the Kt/V formula needs.

• • Single-line percent drop: the result is one number that fits on the same line as the session time and the weight change, used by clinics since the NCDS era.
• • No log or unit conversion: the only inputs are two BUN values, an ultrafiltration volume, and a weight, so the form runs from a routine pre/post-BUN pair.
• • Direct KDOQI 65% benchmark: the adequacy band label maps the percent drop to the National Kidney Foundation 2020 65% cutoff.
• • Pairs with a Kt V upgrade: the same pre/post-BUN values feed a Kt V calculator for the primary spKt/V number.
• • Trended across the month: URR is easy to average across a month and compare with prior months.

The URR percentage is at its strongest when the team trends it across the month and reads it next to a GFR calculator's residual kidney function number and a kidney failure risk estimate.

The URR percentage is a single-session adequacy number, and the Kidney Failure Risk Calculator puts the same clinical picture on a 2-year and 5-year treated-kidney-failure probability for the long-term view.

Five things move the URR percent for a real session, and a few caveats bound what a URR calculator alone can tell the team.

• • URR is a percent drop, not a clearance-per-volume number, so it does not model the nonlinear fall in BUN as well as a Kt/V number. The displayed UF / W is a reference for Kt/V, not an adjustment to URR.
• • A very low pre-BUN can cap the URR even when the session delivered adequate clearance, so the percent should be read next to the pre/post BUN pair, not as a standalone number.
• • Single-session URR is not the KDOQI benchmark; the 65% cutoff is a monthly average, so a single session below 65% is a signal to record in the trend, not an immediate prescription change.

URR and Kt V are the same physical event written two ways, and the percent drop is best read next to the spKt/V from a Kt V calculator on the same BUN pair. For residual kidney function, a GFR calculator on the same serum creatinine is the matching outpatient tool.

As published by the United States Renal Data System 2023, the median delivered URR for US in-center hemodialysis patients sits well above the 65% KDOQI floor; the monthly average can be read against the published distribution.

A widened anion gap and a low URR often travel together, and the Anion Gap Calculator turns the same arterial blood gas into the pH, HCO3, and base-excess labels the team reads next to the URR percent.