Normalizing the peritoneal dialysis dose—have we got it right?

Accuracy of the estimation of V and the implications this has when applying Kt/Vurea for measuring dialysis dose in peritoneal dialysis

Background:

Current guidelines for the prescription of peritoneal dialysis dose rely on a single cut-off ‘minimal’ value of K t/ V. To apply this in the clinic, this requires an accurate estimation of V, the volume of urea distribution that equates to the total body water (TBW). This analysis sought to determine the accuracy to which V can be estimated.

Methods:

A literature search was undertaken of studies comparing TBW estimation using two or three of the following methods: isotopic dilution (gold standard), anthropometric equations (e.g. Watson formula) and bioimpedance analysis. Studies of healthy and dialysis populations of all ages were included. Mean differences and 95% limits of agreement (LOA) were extracted and pooled.

Results:

In 44 studies (31 including dialysis subjects), the between-method population means were typically within 1–1.5 L of each other, although larger bias was seen when applying anthropometric equations to different racial groups. However, the 95% LOA for all comparisons were consistently wide, typically ranging ±12–18% of the TBW. For a typical individual whose TBW is 35 L with a measured K t/ V of 1.7, this translates into a range of K t/ V 1.4–2.05.

Conclusions:

There are limitations to the accuracy of estimation of V which call into question the validity of applying a single threshold K t/ V value as indicative of adequate dialysis. This should be taken into account in guideline development such that if a target K t/ V was deemed appropriate that this should be expressed as a range; alternatively single targets should be avoided and dialysis dose should be determined according to patient need.

A swan song for Kt/Vurea

Dialyzer clearance of urea multiplied by dialysis time and normalized for urea distribution volume (Kt/V_urea or simply Kt/V) has been used as an index of dialysis adequacy since more than 30 years. This article reviews the flaws of Kt/V, starting with a lack of proof of concept in three randomized controlled hard outcome trials (RCTs), and continuing with a long list of conditions where the concept of Kt/V was shown to be flawed. This information leaves little room for any conclusion other than that Kt/V, as an indicator of dialysis adequacy, is obsolete. The dialysis patient might benefit more if, instead, the nephrology community concentrates in the future on pursuing the optimal dialysis dose that conforms with adequate quality of life and on factors that are likely to affect outcomes more than Kt/V. These include residual renal function, volume status, dialysis length, ultrafiltration rate, the number of intra-dialytic hypotensive episodes, interdialytic blood pressure, serum potassium and phosphate, serum albumin, and C reactive protein.