Urea Reduction Ratio (URR) Calculator

How does this urea reduction ratio (URR) calculator work?

This health tool quantifies dialysis accuracy by comparing urea levels before and after dialysis and retrieves a value, known as URR or PRU (Urea reduction percentage).

There are two urea levels required for the calculation:

■ Pre-dialysis urea level;

■ Post dialysis urea level.

Both parameters can be input in five different measurement units: mg/dL, g/L, g/dL, mg% and mg/L.

The formula used in the urea reduction ratio (URR) calculator is:

URR = (U_pre - U_post)/U_pre x 100 = (1 - U_pre/U_post) x 100

Originating in 1991 as a measuring method to correlate dialysis efficiency with patient outcome, the URR permits quick monitoring of dialysis parameters.

In most countries, pre- and post-dialysis urea nitrogen levels are measured on a routine basis, each month or every 12-14 sessions. URR provides insight on the amount of dialysis given when the clearance of urea exceeds the urea generation rate.

Basically, URR shows whether the current dialysis rate is able to remove waste products from the body and this efficiency is expressed in percentage.

Low URRs (lower than 60%) are consistent with higher levels of urea build up and increased bad outcome and this can be worsened by deficitary nutrition.

URR is connected to Kt/V which is one of the main methods by which dialysis measurement is made. There is the following relationship between the two:

(K x t)/V= -ln⁡(1 - URR)

Where:

■ K – dialyzer clearance – blood passage rate in mL/min;

■ t – duration of dialysis;

■ V – volume of bodily water.

Kidney Disease Outcomes Quality Initiative (K/DOQI) national guidelines recommend a Kt/V of at least 1.2 for dialysis efficiency.

Example of a URR calculation

Taking the case of a patient with the following renal function parameters:

■ a pre-dialysis urea level of 65 mg/dL;

■ a post-dialysis level of 20 mg/dL.

The amount of urea removed is 45 mg/dL out of 65 mg/dL meaning 69% (69.23%).

URR = (U_pre - U_post)/U_pre x 100 = (65 mg/dL - 20 mg/dL)/(65 mg/dL) x 100 = 69%

Recommendations are that URRs of 65% and above indicate adequate dialysis.

References

1) Owen WF, Lew NL, Liu Y, Lowrie EG, Lazarus JM. (1993) The Urea Reduction Ratio and Serum Albumin Concentration as Predictors of Mortality in Patients Undergoing Hemodialysis. N Engl J Med; 329:1001-1006.

2) Powers KM, Wilkowski MJ, Helmandollar AW, Koenig KG, Bolton WK. (2000) Improved urea reduction ratio and Kt/V in large hemodialysis patients using two dialyzers in parallel. Am J Kidney Dis. 2000 Feb;35(2):266-74.

3) Kessler E, Ritchey NP, Castro F, Caccamo LP, Carter KJ, Erickson BA. (1998) Urea reduction ratio and urea kinetic modeling: a mathematical analysis of changing dialysis parameters. Am J Nephrol; 18(6):471-7.

4) Press MH, Benz RL. (2006) Quantifying the role of factors that limit attainment of K/DOQI urea reduction ratio dialytic goal. Clin Nephrol; 66(2):98-102.