hen kidneys fail, dialysis is
necessary to remove waste
products such as urea from
By itself, urea is not very
toxic, but its level represents the levels
of many other waste products that build
up in the blood when the kidneys fail.
To see whether dialysis is removing
enough urea, the clinic should periodi-
cally—normally once a month—test a
patient’s blood to measure dialysis ade-
Blood is sampled at the start of
dialysis and at the end.
The levels of
urea in the two blood samples are then
Two methods are generally
used to assess dialysis adequacy, URR
What Is the URR?
The reduction in urea as a result of dial-
ysis, or the URR, is one measure of how
effectively a dialysis treatment removed
waste products from the body.
stands for urea reduction ratio, but it is
commonly expressed as a percentage.
If the initial (predialysis) urea
level was 50 mg/dL and the postdialysis
urea level was 15 mg/dL, the URR is
Although no fixed number can be said
to represent an adequate dialysis, it has
been shown that patients generally live
longer and have fewer hospitalizations
if the URR is at least 60 percent.
this reason, some groups advising on
national standards have recommended
aminimum URR of 65 percent.
The URR is usually measured only once
every 12 to 14 treatments, which is once
a month. It may vary considerably from
treatment to treatment.
For this reason,
a single value below 65 percent should
not be of great concern, but on average
the URR should exceed 65 percent.
What Is the Kt/V?
Another way of measuring dialysis ade-
quacy is the Kt/V.
In this measurement,
K stands for the dialyzer clearance,
expressed in milliliters per minute
(mL/min), and the lowercase t stands
Kt, then, is clearance multi-
plied by time.
This top part of the
fraction represents the volume of fluid
completely cleared of urea during a
If the dialyzer’s clearance is 300 mL/min
and a dialysis session lasts for 180 min-
utes (3 hours), Kt will be 300 mL/min
This equals 54,000 mL, or
In the bottom part of the fraction, V is
the volume of water a patient’s body
The body is about 60 percent
(initial level – postdialysis level)
(50 – 15)
National Institute of Diabetes and Digestive and Kidney Diseases
NATIONAL INSTITUTES OF HEALTH
U.S. Department of Health
and Human Services