File:Urr ktv.svg

Summary
Nomogram showing the relationship between hemodialysis Kt/V and URR (urea reduction ratio), using the 2nd generation logarithmic estimate, as described in Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error J Am Soc Nephrol. 1993 Nov;4(5):1205-13. . The nomogram allows one to estimate Kt/V from the URR and the fractional weight loss (UF/W) during a dialysis treatment. UF (ultrafiltrate volume) is equivalent to the weight change during the dialysis (in lbs or kg), and W is the postdialysis weight (in lbs or kg). Typical weight loss during a dialysis session is 2 kg for a 70 kg patient, or 3% of the body weight (UF/W = 0.03) as shown by the green line.

The dashed red line represents the relationship between URR and Kt/V in the absence of urea generation or weight loss. Under those conditions, a URR of 0.63 (or 63%) would be expected when Kt/V = 1.0 (heavy black lines). The next line to the right (in light blue) shows the effects of urea generation during a typical 3-4 hour treatment. Urea generation during such a session results in a slightly lower URR (by about 0.10) than expected as a result of urea clearance, due to the fact that urea is being added to the body by generation during the dialysis session. The UF/W = 0.03, 0.06, and 0.09 isopleths represent the relationship with differing amounts of volume contraction. With volume contraction, more urea is cleared at a given level or URR. It is the same as removing dust from a house that shrinks while it is being cleaned. Urea kinetics standardizes on the post-dialysis volume (V). So if one removes 50% of the dust from two houses, and their final sizes are the same, but the initial size of one house was larger, then more dust will have been removed from the larger house. The heavy green line shows the relation when UF/W - 0.03. So in practice, because of urea generation and UF/W, a 63% URR (0.63) results in a Kt/V of about 1.15 instead of only 1.0 (see graph).

KDOQI hemodialysis adequacy standards are written in terms of either Kt/V or URR, with Kt/V being the preferred choices. But a patient may meet one standard and not the other, depending on the level of UF/W. When little fluid is removed, a given URR value will have a lower Kt/V value then when a large amount of fluid relative to postdialysis body weight (W) has been removed.

Such a nomogram will be slightly less accurate when very long dialysis sessions are used, since it assumes a dialysis session length of 3-4 hours.