What this calculator does
Dialysis adequacy is measured by how much waste a session actually removes, not by how long the machine ran. The urea reduction ratio compares blood urea before and after a session, using urea as a proxy for waste clearance generally.
The conventional threshold is 65 per cent. A pre-dialysis urea of 60 mg/dL falling to 18 gives 70.0 per cent, which clears it, while falling only to 25 gives 58.3 per cent, which does not.
The formula
Subtract the post-dialysis urea from the pre-dialysis figure and divide by the pre-dialysis figure. Both samples come from the same session.
| Term | Meaning |
|---|---|
| URR | Urea reduction ratio: the percentage fall in urea across a session. |
| Adequacy threshold | 65 per cent or above is generally considered adequate. |
| Kt/V | A related and more elaborate adequacy measure that accounts for volume and time. |
The inputs explained
| Field | What to enter |
|---|---|
| Pre-dialysis BUN (mg/dL) | Pre-dialysis blood urea nitrogen in mg/dL. |
| Post-dialysis BUN (mg/dL) | Post-dialysis blood urea nitrogen, drawn according to the unit's sampling protocol. |
When to use it
Checking dialysis adequacy
URR is measured routinely, typically monthly, as a quality measure.
Investigating why a patient feels unwell
Inadequate clearance is one possible contributor.
Understanding a dialysis report
The percentage follows directly from two blood samples.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What post-dialysis level clears the threshold?
Three post-dialysis levels from the same starting point.
| Post-dialysis urea | Urea reduction ratio | Reading |
|---|---|---|
| 18 mg/dL | 70.0% | At or above the usual adequacy threshold |
| 25 mg/dL | 58.3% | Below the usual adequacy threshold: worth discussing with the treating team |
| 30 mg/dL | 50.0% | Below the usual adequacy threshold: worth discussing with the treating team |
Questions
How does URR compare with Kt/V?
Kt/V accounts for the patient's urea distribution volume and for urea generated during the session, so it is more complete. URR is simpler and correlates closely, which is why both are reported and a URR of 65 per cent roughly corresponds to a Kt/V of 1.2.
Why does the timing of the post sample matter?
Because urea rebounds from tissues into the blood after dialysis stops. Drawing too late gives a falsely low reduction, which is why units follow a strict sampling protocol, usually within a defined window after the session ends.
Does a high URR mean better outcomes?
Up to the threshold, clearly. Beyond it the evidence is weaker, and the HEMO trial found no survival benefit from dialysis doses substantially above standard targets.
Does urea removal capture everything?
No. Urea is a small molecule that clears readily, and it stands in poorly for larger uraemic toxins and for phosphate. Adequacy by URR does not guarantee adequate clearance of everything else.
For kidney filtration estimates, see the creatinine clearance calculator. For the blood urea ratio, see the BUN-to-creatinine ratio calculator.