Accuracy of the estimation of V and the implications this has when applying K t / V urea 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.