About the URR Calculator - Urea Reduction Ratio
The Urea Reduction Ratio (URR) is a simple, widely used measure of hemodialysis adequacy. It compares the blood urea nitrogen (BUN) level drawn just before a dialysis session to the BUN level drawn just after, expressing the drop as a percentage. This calculator also estimates single-pool Kt/V, a related and more complete adequacy measure, from the same BUN values plus session length, ultrafiltration volume, and post-dialysis weight.
The formula
URR is calculated as:
URR = (Pre-dialysis BUN - Post-dialysis BUN) / Pre-dialysis BUN x 100
Both BUN values must use the same units (typically mg/dL). A pre-dialysis BUN of 60 mg/dL and a post-dialysis BUN of 18 mg/dL gives a URR of (60-18)/60 x 100 = 70%.
Estimating Kt/V from the same values
Kt/V accounts for fluid removed during treatment and urea generated during the session, which URR does not capture. This tool applies the Daugirdas second-generation formula for single-pool Kt/V:
Kt/V = -ln(R - 0.008 x t) + (4 - 3.5 x R) x UF / W
where R is the post/pre BUN ratio, t is session length in hours, UF is ultrafiltration volume in liters, and W is post-dialysis weight in kilograms.
Putting the result in context
National kidney guidelines (KDOQI) commonly set a minimum adequacy target of URR at least 65%, with many dialysis programs targeting 70% or higher on a standard three-times-weekly schedule, corresponding to a single-pool Kt/V around 1.2 to 1.4. A result below these thresholds is a signal to review treatment time, blood or dialysate flow, and vascular access function - not a diagnosis on its own.
When to consult a professional
This calculator performs the standard URR and Kt/V arithmetic for education and record-keeping. It is not a substitute for clinical assessment. Dialysis prescriptions and any changes to treatment time, frequency, or access should be decided with a nephrologist or the dialysis care team, who can account for residual kidney function, comorbidities, and lab timing.