Current guidelines for the prescription of peritoneal dialysis dose rely on a single cut-off 'minimal' value of /. To apply this in the clinic, this requires an accurate estimation of , the volume of urea distribution that equates to the total body water (TBW). This analysis sought to determine the accuracy to which can be estimated.

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.

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 / of 1.7, this translates into a range of / 1.4-2.05.

There are limitations to the accuracy of estimation of which call into question the validity of applying a single threshold / value as indicative of adequate dialysis. This should be taken into account in guideline development such that if a target / 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.