When kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. Urea is not very toxic by itself, but its level represents the levels of many other waste products that build up in the blood when the kidneys fail.
Blood is sampled at the start of dialysis and at the end.
The levels of urea in the blood in the two samples are then compared. Although there is no fixed number that represents an adequate dialysis, it has been shown that patients generally live longer and have fewer hospitalizations if the URR is at least 60 percent to 70 percent or higher. For this reason, some national standards groups have recommended a minimum URR of 65 percent. For this reason, a single value that is below 65 percent should not be of great concern, but on average, the URR should exceed 65 percent. Kt, then, is clearance multiplied by time.