Urea Reduction Ratio (URR)

Calculates the Urea Reduction Ratio, a simple measure of dialysis adequacy. URR >= 65% corresponds to adequate dialysis in most patients.

mg/dL (or any consistent unit)
Pre-dialysis BUN. Any unit works as long as pre and post use same unit.
mg/dL (same as pre)
Post-dialysis BUN using same units as pre-dialysis.

References

  1. Urea reduction ratio may be a simpler approach for assessing adequacy of intermittent hemodialysis in AKI — BMC Nephrology/PMC (2019)
  2. Reference equations for the six-minute walk in healthy adults — American Journal of Respiratory and Critical Care Medicine (1998)

Note: Relationship to Kt/V: (K*t)/V = -ln(1 - URR). URR underestimates solute clearance associated with ultrafiltration but is validated for IHD in AKI.

Warnings & Limitations

  • URR is simpler but less precise than Kt/V. URR does not account for ultrafiltration clearance.
  • Reliability reduced when pre-dialysis BUN < 20 mg/dL.
  • URR >= 65% generally corresponds to spKt/V >= 1.2.
  • URR >= 67% corresponds to Kt/V >= 1.2 with >99% accuracy (ATN study).
  • Does not apply to peritoneal dialysis or CRRT.