The main obstacle in defining the equivalent doses in HD and PD is the fact that PD is usually a continuous process while HD is intermittent. The Kt/V concept was introduced by Gotch and Sargent to quantitate dialysis dose and represents the product of urea clearance (K) and dialysis time (t) divided by the volume of distribution of urea or total body water (V)1. Kt/V is different in HD compared to PD. In HD, Kt/V relates to a single dialysis session, whereas in PD Kt/V relates to an entire week.
The logarithmic (ln) term in the HD Kt/V equation is used to account for the fall in efficiency as urea concentrations diminish during the HD session. In CAPD, this fall in efficiency does not happen and Kt/V relates to the actual mass of urea removed. This means that Kt/V cannot be compared between HD and PD since they are completely different measures which happen to share the same name.
HD (per session) Kt/V = ln (pre/post)
The solute removal index (SRI) has been proposed to overcome the confusion with Kt/V. SRI is the ratio of mass of urea removed per week to peak mass of urea in the patient2. SRI is applicable equally to HD, PD and renal function. Patients with the same urea generation rate and SRI will have the same peak urea concentration, regardless of treatment type. SRI assumes that toxicity is related to the peak urea concentration. SRI is equivalent to the weekly Kt/V in PD. In HD, it is similar to the urea reduction ratio multiplied by the number of treatments per week and should account for ultrafiltration, urea generation and the post-dialysis rebound for more precision.
Casino and Lopez proposed an alternative method of quantifying dialysis of different types and frequencies3. The dialysis is quantified as equivalent renal clearance (EKRU) in ml/min/1.73m2. The EKRU is calculated in the same way as for normal renal urea clearance (mass urea removed/mean blood urea concentration/time). EKRU is similar to SRI, but relates mass removed to the average urea concentration rather than the peak. Also EKRU normalizes to surface area rather than V.
Based on these concepts, but defining peak urea as a midweek urea concentration, Gotch introduced the standard Kt/V (stdKt/V) concept to measure the relative efficiency of the whole spectrum of dialytic therapies whether intermittent, continuous or mixed4.
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The stdKt/V is a method to measure the clearance efficiency of HD of variable frequency, continuous PD, intermittent PD, slow continuous renal replacement therapies (CRRT), residual renal function (RRF) and all situations involving the use of clearance from various methods5.
References:
- Gotch F, Sargent JA. A mechanistic analysis of the National Cooperative Dialysis Study (NCDS). Kidney Int 28:526-534, 1985
- Keshaviah P. The solute removal index--A unified basis for comparing disparate therapies. Perit Dial Int 15:101-104, 1995
- Casino FG, Lopez T. The equivalent renal urea clearance: A new parameter to assess dialysis dose. Nephrol Dial Transplant 11:1574-1581, 1996
- Gotch FA. The current place of urea kinetic modeling with respect to different dialysis modalities. Nephrol Dial Transplant 13:10-14, 1998
- Diaz-Buxo JA, Loredo JP. Standard kt/v: comparison of calculation methods. Artif Organs 30:178-185, 2006