The use of bioelectric impedance analysis to measure fluid compartments in subjects with chronic paraplegia.

Abstract

To determine the sensitivity and specificity of body mass index (BMI) as a surrogate marker of obesity in individuals with chronic paraplegia and to validate bioelectric impedance analysis (BIA) as a method of measuring body composition in this group. Cross-sectional study. University hospital. Convenience sample of 31 subjects with paraplegia (19 men, 12 women; mean age, 34.2+/-8.8y) and 62 able-bodied control subjects (30 men, 32 women; mean age, 28.6+/-7.2y). Not applicable. Total-body water (TBW) by deuterium dilution; extracellular water (ECW) by corrected bromide space. Fat-free mass (FFM)=TBW/.732; fat mass (FM)=weight-FFM. Single-frequency whole-body and segmental BIA, and multifrequency whole-body BIA. BMI had 100% specificity and 20% sensitivity in distinguishing obese from nonobese subjects with paraplegia. TBW was predicted by using the equation: TBW (inL)=2.11-0.1age+3.45sex+.34wt+.28(ht(2)/R)-.086sex x wt(r(2)=.95, standard error of the estimate [SEE]=1.86L, P<.0001). This equation had 81.8% specificity and 68.4% sensitivity. ECW was predicted by using the equation: ECW (in L)=-.025+1.03sex+.187wt+.0041(ht(2)/X(c)) -.033sex x wt (r(2)=.75, SEE=1.62L, P<.0001). Multifrequency BIA offered no greater prediction of TBW or ECW than single-frequency BIA. BMI has excellent specificity but poor sensitivity in distinguishing obese from nonobese individuals with paraplegia. TBW (and therefore FFM and FM) and ECW can be reasonably well predicted by using single-frequency BIA.