Limitations in anthropometric calculations of total body water in patientson peritoneal dialysis

Having an accurate estimation of total body water (TBW) is essential for th e evaluation of dialysis efficacy in peritoneal dialysis (PD) patients. In this study, TBW volumes were measured by tritium dilution (TBWTHO) in 165 P D patients and compared with TBW calculations according to the Watson formu las. An alternative anthropometric formula based on the present PD populati on was also developed and validated in an independent sample of 29 PD patie nts. Furthermore, the relation between TBWTHO and body surface area (BSA) a ccording to the formula of Gehan was analyzed. Body composition was assesse d by a four-compartment model, based on measurements of TBWTHO and total bo dy potassium. Mean values of TBW by the Watson formulas were almost identic al to TBWTHO, and the correlation coefficient for the relationship of calcu lated to measured volumes was 0.89 (P = 0.001). However, both anthropometri c formulas-the Watson formulas as well as the new, alternative one-overesti mated TBW in obese patients and vice versa in lean patients. Similarly, TBW was underestimated in patients who were overhydrated. The correlation coef ficient between TBWTHO and BSA was 0.708 for males and 0.797 for females (P = 0.0001 for both). In obese patients, the relationship was even closer (r = 0.924 and 0.911, respectively). In conclusion, anthropometric formulas t o calculate TBW showed a considerable intraindividual variability compared with measured values. This was related to body composition features, such a s degree of obesity and hydration. In contrast, BSA correlated closely to T BW in obese individuals. TBW as estimated by anthropometric formulas must b e analyzed with caution, especially in the very obese or very lean patient.