How this calculator works
Use pre- and post-dialysis results collected according to the dialysis unit sampling protocol. Their units and analyte convention must match. URR describes the proportional fall in blood urea; it does not account for urea generation, ultrafiltration or post-dialysis rebound. A care team interprets adequacy for the prescribed schedule and residual kidney function. The result is an estimate for information only and is not medical advice.
Formula
URR = 1 - post / pre. Multiply by 100 to express a percentage. Pre = post / (1 - URR); post = pre × (1 - URR), with URR expressed as a fraction in the inverse equations.
Worked example
A fall from 60 to 20 mg/dL gives 1 - 20/60 = 66.6667%. A fall from 80 to 20 gives 75%.
Advertisement
Frequently asked questions
Is URR the same as Kt/V?
No. Daugirdas single-pool Kt/V includes additional duration and fluid removal terms.
Why do sampling times matter?
Blood from the access and subsequent urea rebound can change the measured post-dialysis value.
Can URR be 100%?
The arithmetic allows a measured post value of zero when calculating URR or post concentration, but that boundary cannot recover a unique pre value.