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Body composition, hydratation status and ultrafiltrate per session evaluated by bioelectrical impedance and dual X-ray absorptiometry in hemodyalisis.

Atilano Ximena  Correa Rotter R  Espinosa MA

Poster presented at the American Society of Nephrology Congress (Renal Week) at Philadelphia, USA November 2005.

INTRODUCTION: Patients in hemodyalisis programs present constant fluctuations in their hidratation status, therefore it's important to evaluate total body water and dry weight, in virtue of it's relation with the dialitic prescription and cardiovascular complications. The dual X-ray absorptiometry (DXA) is consider as the gold standard to estimate body composition. The bioelectrical impedance (BIA) is a simple method, that can be use in a rapid and bedside manner at low cost that has demonstrated to be useful to evaluate the body composition, nevertheless, it has been questioned it's accuracy when compared to DXA in subjects with water retention. On other hand, the net ultrafiltrate volume per hemodyalisis session can be estimated by different methods, but the accuracy of these has not been evaluated.

a) Compare the results of body composition and hidratation status determination by DXA (gold standard) and BIA in hemodyalisis patients.
b) Compare the ultrafiltrate volume estimation by DXA, BIA and other conventional methods to calculate dry weight against the cuantitative determination of it.

MATERIAL AND METHODS: Body composition was evaluated in 10 hemodyalisis patients, 6 women and 4 men, between 18 and 70 years, by pre and post hemodyalisis measurements with DXA and BIA (Bodystat QuadScan 4000, UK). The results were compared with those obtained with the traditional formulas use to predict total body water, at the same time, the ultrafiltrate volume in each hemodyalisis session was compared with the different estimations of total body water.

RESULTS: Both, the fat free mass (FFM) and total body water (TBW) estimated by BIA, significantly decrease in the postdyalisis period, while the fat mass increase, in contrast to the determinations by DXA in which the bone mineral content and fat mass keep stable and fat free mass decrease after the dialitic treatment. Considering DXA as the gold standard to measure total body water, we found that BIA and the 73% of the body weight overestimate water content, while the weight percentage, the Hume and Watson's formulas underestimate it, while in all the estimations, the total body water is significantly less in the postdyalisis period by all methods (Table 1). The methods that best correlated the total body water estimations by DXA in the postdiálisis period were the percentage of body weight, the Hume and the Watson's formula, but in the postdiálisis period the best correlation was made by BIA. The only determinations in which no significant differences in the ultrafiltrate were found are those obtained by BIA, it means that the difference in the pre and postdyalisis determinations of body water by BIA are similar than the liters of ultrafiltrate, in contrast to that obtained by the other methods (Table 2). The correlations between the prediction formulas and the ultrafiltrate are very good but the BIA is the only method in which the concordance is adequate (R2=0.516 p=0.01, Kappa=0.48 p=0.0004).

CONCLUSIONS: Different to BIA, DXA is an accurate method to estimate the body composition an hydratation status in patients with water retention. The conventional BIA is an acceptable method to estimate the amount of ultrafiltrate in hemodyalisis patients. The 73% of the body weight is the least accurate equation to determinate total body water.

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