Abstract: Urea clearance, adjusted for total body water using the Watson formula (Kt/Vurea) is widely used to guide peritoneal dialysis prescription and ensure dialysis adequacy. The impact of body composition on determining body water using the Watson formula (TBWWatson) is well established, but the effect of hydration on TBWWatson and Kt/V is not well understood. We therefore in the present study tried to estimate the magnitude of errors in using Kt/V to assess dialysis adequacy in peritoneal dialysis patients. 195 stable adult peritoneal dialysis patients were enrolled. Total body water was measured by multiple-frequency bioelectrical impedance spectroscopy (MF-BIS) and compared with the TBW calculated using the Watson formula. Furthermore we calculated Kt/V by two methods — one is normalized to total body water according to Watson formula and the other is normalized to total body water according to MF-BIS. Patients were divided into three groups according to their degree of overhydration (Δhydration status (OH, L)): normally hydrated group (n=101, OH4.0L). Compared with MF-BIS, the Watson formula overestimated TBW in normally hydrated patients but underestimated TBW in severely overhydrated patients. On the other hand, compared with the Kt/V normalized to total body water according to MF-BIS, the Kt/V normalized to total body water according to Watson formula clearly underestimated Kt/V in normally hydrated patients but overestimated Kt/V in severely overhydrated patients. Our study suggested that hydration status strongly affect total body water estimation by the Watson formula in peritoneal dialysis patients, potentially leading to inappropriate dialysis prescription and failure to achieve recognized Kt/Vurea adequacy targets. Key words: bio-impedance, peritoneal dialysis, hydration status, Watson formula, dialysis adequacy

Key words: bio-impedance, peritoneal dialysis, hydration status, Watson formula, dialysis adequacy