Since residual renal clearance (Kru) is continuous, and dialysis clearance is intermittent (with Kt/V referring to clearance during a single dialysis session), the quantities of both cannot simply be added. When it is planned to reduce dialysis dose by incorporating the contribution of residual function, it is necessary to somehow add these clearances so that the dialysis component is appropriate. Three methods of combining these clearances have been proposed.
Converting Kru to an equivalent eKt/V (Combined eKt/V)
In this method, residual kidney function is converted to an equivalent per session eKt/V, by multiplying by a factor F, which empirically inflates the time over which residual clearance is measured, to account for its greater efficiency compared to that of dialysis, with the value of F depending on dialysis frequency. A “Combined Kt/V” is calculated:
Combined eKt/V = eKt/VDialysis + eKt/VKidney
eKt/VKidney = Kru * F / Vu
where eKt/VDialysis is calculated as above
Vu = volume of distribution of urea (ml), approximated by 580 * TW (kg)
F = 5500 (for thrice weekly schedules)
With the method above, a Combined eKt/V can be calculated and the dialysis component adjusted to achieve a target Combined eKt/V of 1.2.
Converting Kt/V to an equivalent renal clearance (EKRc)
An alternative method is to convert per session Kt/V into an equivalent continuous renal clearance and then add it to Kru. Casino and Lopez computed kinetic estimates of combined dialysis and kidney urea clearance (normalized to volume) which they termed “equivalent renal urea clearance” (EKRc). In the absence of residual function, an eKt/V target of 1.2 equates to EKRc 13ml/min. For a thrice weekly schedule EKRc is given by the formula:
EKRc (ml/min) = 1 + (10 * eKt/V)
With this method Kru is added to EKRc and the dialysis component adjusted to achieve a total of 13ml/min.
Converting both eKt/V and Kru to a weekly dialysis dose (stdKt/V)
Dialysis schedule and dose can be converted to an equivalent weekly clearance, “Standard Kt/V” (stdKt/V), proposed by Gotch, based on kinetic models which relate urea generation to average weekly pre dialysis urea. This allows comparison between schedules: a frequent schedule with stdKt/V = 2.1 is equivalent (in terms of small solute clearance) to a thrice weekly schedule with eKt/V = 1.2. Residual function can be incorporated into stdKt/V (sometimes termed “Total Standard Kt/V”) by formulas available.
Reference
- Daugirdas JT, Depner TA, Greene T et al. Standard Kt/Vurea: a method of calculation that includes effects of fluid removal and residual kidney clearance. Kidney Int 2010; 77: 637 644