Wednesday, February 23, 2011

Measures of dialysis dose

As previously discussed on RFN, the urea Kt/V is a measure of dialysis dose that is related to patient outcome. There are several different Kt/Vs encountered in the dialysis literature.

spKt/V = single pool
eKt/V = equilibrated
stdKt/V = weekly standard

A nice way to think about each one is in the context of the major trials in which they were used.


The National Cooperative Dialysis Study (NCDS) published in 1981 examined four different 3x week dialysis prescription targets in 151 patients. The original paper did not use Kt/V. Instead, the prescription targets were high vs low time averaged BUN and long vs short dialysis treatment times.

The time averaged BUN and dialysis times achieved were approximately 90 mg/dl vs 50 mg/dl and 4.5 hours vs 3.25 hours in the high vs low and long vs short groups respectively. Protein intake was not randomized and was meant to be between 0.8 and 1.4 g/kg though some patients fell below this range.

The study showed that patients in the high BUN groups were hospitalized and withdrawn from the study protocol at statistically significant higher rates. Time was not a statistically significant variable for either of these outcomes though the p value for increased risk of hospitalization in the short time group was 0.06. The original NCDS paper did not sort out whether people in the each of the BUN groups were there because of their dialysis dose or because of their protein intake.

A subsequent reanalysis in KI by Gotch and Sargent in 1985 separated these variables out using the single pool Kt/V (for dose) and normalized protein catabolic rates (for protein intake). As seen below they showed that poor outcomes were associated with a spKt/V of less than 1.0 in 3x per week dialysis.

In practice the spKt/V is calculated for a single run of dialysis using known variables as inputs in any of several developed equations. The commonly used Daugirdas equation…

spKt/V = -ln(R - 0.008*t) + (4 - 3.5*R)(preBW-postBW/preBW)

Uses the known variables of…

R (post dialysis BUN/pre dialysis BUN)
preBW (pre dialysis body weight)
postBW (post dialysis body weight)
t (treatment time)

The spKt/V is used in most dialysis units to assess dose for patients on 3x week dialysis schedules. There are however, other versions of Kt/V that are seen in the literature and are useful in certain situations. On that note, stay tuned for eKt/V…

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