title: Comparison of bioimpedance and dual-energy x-ray absorptiometry for measurement of fat mass in hemodialysis patients.
abstract: BACKGROUND
  Fat mass (FM) is measured with dual-energy X-ray absorptiometry (DXA), but is expensive and not portable. Multifrequency bioimpedance spectroscopy (BIS) measures total body water (TBW), intracellular water (ICW) and extracellular water (ECW). FM is calculated by subtracting fat-free mass (FFM) from weight assuming a fractional hydration of FFM of 0.73. Hemodialysis (HD) patients, however, have nonphysiologic expansion of ECW. Our aim was to apply a model to estimate FM in HD patients and controls.
  
  
  METHODS
  We estimated the hydration of FFM in healthy subjects and HD patients with BIS (Impedimed multifrequency) assuming a hydration of 0.73 or using a model allowing ECW and ICW to vary, deriving a value for FM accounting for variances in ECW and ICW. FM was measured by DXA (Hologic Discovery W) in 25 controls and in 11 HD patients. We measured TBW, ECW and ICW with BIS and calculated FM using either weight - TBW/0.73 or with a model accounting for variations in ECW/ICW to estimate FM.
  
  
  RESULTS
  ECW/ICW was greater in HD patients than in controls (0.83 ± 0.08 vs. 0.76 ± 0.04; p = 0.001). FM (kg) measured by DXA or estimated from TBW using constant hydration or accounting for variations in ECW/ICW was not significantly different in controls or in HD patients. Values obtained by all methods correlated (p < 0.001) and none of the Bland-Altman plots regressed (r(2) = 0.00). FM measured by DXA and by BIS in both controls and HD patients combined correlated (r(2) = 0.871).
  
  
  CONCLUSIONS
  Expansion of ECW in HD patients is statistically significant; however, the effect on hydration of FFM was insufficient to cause significant deviation from values derived using a hydration value of 0.73 within the range of expansion of ECW in the HD patient population studied here.
authors.affiliation: Department of Internal Medicine, University of California, Davis, Calif., USA.
authors.forename: Alessio
authors.initials: A
authors.lastname: Molfino
authors.forename: Burl R
authors.initials: BR
authors.lastname: Don
authors.forename: George A
authors.initials: GA
authors.lastname: Kaysen
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods*; D000273:Adipose Tissue / Q000503:physiopathology*; D050154:Adiposity; D000328:Adult; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000175:diagnosis / Q000503:physiopathology* / Q000534:rehabilitation*; D008297:Male; D008875:Middle Aged; D010992:Plethysmography, Impedance / Q000379:methods*; D006435:Renal Dialysis; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome