Body Composition Affects Urea Distribution Volume Estimated by Watson's Formula.

Objective: Dialysis machines use the Watson formula (Vwatson) to estimate the urea distribution volume (UDV) to calculate the online Kt/V for each dialysis session. However, the equation could give rise to inaccuracies. The present study analyzes whether body composition affects UDV estimated by Vwatson in comparison to bioimpedance spectroscopy (Vbis) as the reference method.

Design: This is a transversal study performed in the setting of a hemodialysis unit.

Subjects: Prevalent hemodialysis patients.

Intervention: The same day, UDV was measured using Vwatson and Vbis. We compared their results.

Main Outcome Measure: Differences between UDV using Watson equation and Vbis.

Results: We included 144 prevalent patients. Vwatson overestimated the volume with regard to Vbis (Vwatson - Vbis) by 2.5 L (1.8 L; P = .001). We found an excellent correlation between the 2 methods. A higher mean Vwatson - Vbis value was correlated to older age (P = .03), body mass index (P = .01), fat tissue index (P = .001), lower lean tissue index (P = .001), lower extracellular water (P = .01), and intracellular water (P = .001).

Conclusion: Body composition affects UDV estimated by Vwatson, thus modifying the result of Kt/V. In young patients who present more lean tissue and less fat tissue, Kt/V is underestimated with Vwatson.