So your anuric patient has chosen home hemodialysis. How do you go about creating an initial prescription? In this post we’ll discuss an approach that should serve as a good starting point.
Step 1. Pick your target per-treatment spKt/V.
Setting aside controversy around urea-based adequacy measures, validation, and goal stdKt/V for home hemodialysis, let’s assume a reasonable starting target stdKt/V of 2.0. This corresponds to a per-treatment spKt/V goal of 0.5 for 6x/week HD, and 0.6 for 5x/week HD.
Your patient intends on 5 sessions per week, so goal spKt/V is 0.6 per treatment.
Step 2. Determine how many liters of dialysate you need to fully saturate with urea to achieve an adequate per-treatment spKt/V.
Estimate TBW using the Watson equation.
Suppose height 170cm, weight 70kg, male, age 60.
Using the calculator, TBW = 39L
39L * 0.6 = 23.4L of fully saturated dialysate needed to achieve spKt/V 0.6.
Let’s round up to 24L – always avoid erring on the side of underdialysis!
Step 3. Pick a flow fraction (FF). FF is defined as Qd/Qb. With in-center HD, high Qd prevents dialysate from being saturated and ensures a large gradient for solute removal is maintained, so in practice the main determinant (and limiting factor) of clearance is Qb. This approach minimizes treatment time. But because of the huge volume of water required for this, this is impractical for home HD: do the math; a conservative prescription of 180 minutes and Qd = 0.6L/min already uses more than 100L!
The alternative is the Low Dialysate Volume Approach. By using slower Qd, home HD relies on maximizing dialysate saturation to permit as efficient a use of water as practical. As it turns out, a useful property is that saturation depends on FF. Whether it’s a Qd of 500 and Qb of 250 (FF=2) or a Qd of 800 and Qb of 400 (FF=2), saturation is the same!
Because we’re now prescribing based on saturation, we exploit this property to simplify our prescription. If we set a FF, we know the % saturation, and the question then becomes how many liters of dialysate we need to saturate at that % to achieve our goal Kt/V.
Typical FF for home HD is 0.2 to 0.5. Higher FF means less saturation. Useful rules of thumb are that a FF of 0.2 gives 95% urea saturation, 0.4 gives 90%, and 0.5 gives 85%.
Let’s set FF=0.5 for our patient.
Step 4. Calculate the actual necessary volume of dialysate. Knowing the target in liters of fully saturated dialysate and the actual % saturation our chosen FF will achieve, the math works itself out.
24L at 100% saturation = 28.2L at 85% saturation
(math: 24/0.85=28.2)
The key insight and beauty of FF is that once you set it, and you set the total dialysate volume, you’re guaranteed the desired clearance no matter how long or short the treatment is, or no matter what the Qb is.
Current technology (though this may change) prepares dialysate as 50L or 60L batches, so prescribing a volume that 50 or 60 is divisible by will prevent waste. So instead of our 28.2L prescription, we’ll go to 30L.
How long will this treatment take?
Prescribing the FF, Qb, and total dialysate volume makes time a dependent variable.
FF = 0.5, Qb = 400
Qd = FF*Qb = 200 mL/min
30L at 200 mL/min = 30L / 0.2L/min = 150 min
Or: Time = total dialysate volume / (FF*Qb)
If I’ve set FF and total dialysate volume, it means I can shorten/lengthen treatment time by increasing/decreasing Qb. Ordinarily Qb would be determined by vascular access, and would not commonly be available as a way to eke out additional clearance (when combined with increasing dialysate volume or decreasing FF) if we are concerned about inadequate dialysis.
As middle molecular clearance is time dependent, a minimum of 12 hours per week is used to ensure adequate middle molecule clearance. Here 150 min * 5 treatments = 12.5 hours/week, so we pass this check.
So how do I increase the dialysis dose?
Ways to increase clearance include increasing total dialysate volume or decreasing FF. Both will increase treatment time. But because of their relationship with time, there may be a way to increase volume and also increase FF, in a manner that improves clearance while minimizing impact on time.
Final Thoughts
We purposely didn’t account for ultrafiltration as a mode of clearance. By ensuring our prescription is adequate without UF, we err on the side of over-dialysis once UF is taken into account.
If a patient isn’t anuric, residual kidney function Kt/V can be subtracted from the goal weekly stdKt/V. As a rule of thumb, each 1 mL/min of GFR is about 10L clearance (Kt) per week. So if GFR = 3mL/min, that’s 30L, and if TBW = 40L, that’s a Kt/V of 0.75–so now the home HD calculations can start with a weekly stdKt/V of 1.25 instead of 2, but you’d have to re-consult the charts to see what per-treatment spKt/V that would correspond to for 5x/week or 6x/week HD.
Finally, Kt/V is not the only measure of adequacy. Fluid balance, electrolyte homeostasis, and clinical uremia continue to be important parameters for assessing the adequacy of renal replacement therapy!
Chi Chu, MD
Nephrology Fellow,
University of California-San Francisco
Ankur Shah, MD
Assistant Professor of Medicine,
Brown University
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whole lot and never manage to get nearly anything done.