Impact of hydration and nutrition status on the Watson formula in peritoneal dialysis patients

Urea clearance (Kt/V urea), adjusted for total body water (TBW) using the Watson formula (TBW(Watson)), is widely used to guide peritoneal dialysis (PD) prescription and to ensure dialysis adequacy. The impact of body composition on the determination of TBW(Watson) is well established, but the effect of hydration and nutrition status on TBW(Watson) is not understood. We therefore studied the effects of hydration and nutrition status on TBW(Watson) in PD patients. Our study enrolled 195 PD patients and 33 healthy control subjects. Multiple-frequency bioelectrical impedance spectroscopy (MF-BIS) was used to measure TBW and the result was compared with TBW(Watson). Patients were divided into three groups according to their degree of overhydration [deltahydration status (OH) in liters]: normally hydrated group (OH: < 2.0 L), mildly overhydrated group (OH: 2.0 - 4.0 L), and severely overhydrated group (OH: > 4.0 L). Compared with MF-BIS, the Watson formula overestimated TBW in normally hydrated patients, but underestimated TBW in severely overhydrated patients. In addition, of the normally hydrated patients, 22 were malnourished by subjective global assessment, and the TBW(Watson) overestimation was much greater in them than in the well-nourished patients. Our study suggests that hydration and nutrition status both strongly affect TBW(Watson) in PD patients.