DEVELOPMENT OF A POPULATION-SPECIFIC REGRESSION EQUATION TO ESTIMATE TOTAL-BODY WATER IN HEMODIALYSIS-PATIENTS

We have previously shown that the impedance index (height corrected re sistance) is a valid and reliable correlate of total body water (TBW) in hemodialysis patients. We estimated TBW by single frequency bioelec trical impedance analysis (BIA) in 3009 in-center hemodialysis patient s, and developed an ESRD-specific TBW equation from routinely availabl e demographic and anthropometric variables. The mean +/- SD age was 60 .5 +/- 15.5 years; 47% were female. 47% African-American, and 36% diab etic. Dialysis duration was 3.8 +/- 3.7 years. Mean TBW was 40.8 +/- 9 .3 kg, 56 +/- 9% of body weight, A stepwise linear regression equation was fit on a two-thirds random sample, deriving significant parameter estimates for the variables age, gender, height, weight, diabetic sta tus, weight squared, and the cross-products of age and gender, age and weight, gender and weight, and height and weight. The equation was th en validated in the remaining one-third sample, and compared with TEW estimates by the Watson and Hume-Weyer formulae. TBW estimated by our equation (40.6 +/- 8.6 kg) was not significantly different from the BI A TEW (40.5 +/- 9.3 kg). In contrast, TBW estimated by the Watson (37. 0 +/- 7.6 kg) and Hume-Weyer (37.9 +/- 7.7 kg) formulae underestimated TEW by a mean of 3.5 and 2.6 kg, respectively. A population-specific equation provides superior prediction of TEW in hemodialysis patients. The use of formulae developed and validated in non-uremic populations may result in underestimates of TEW in patients with ESRD, and potent ially, overestimates of dialysis dose approximated by the clearance-ti me to TEW ratio (Kt/V).