Evaluation of DXA against the four-component model of body composition in obese children and adolescents aged 5 to 21 years

BACKGROUND: Body composition is increasingly measured in pediatric obese patients. Although dual-energy X-ray absorptiometry (DXA) is widely available, and is precise, its accuracy for body composition assessment in obese children remains untested. OBJECTIVE: We aimed to evaluate DXA against the four component (4C) model in obese children and adolescents in both cross-sectional and longitudinal contexts. DESIGN: Body composition was measured by DXA (Lunar Prodigy) and the 4C model in 174 obese individuals aged 5-21 years, of whom 66 had a second measurement within 1.4 years. The Bland-Atman method was used to assess agreement between techniques for baseline body composition and change therein. RESULTS: A significant minority of individuals (n = 21) could not be scanned successfully due to their large size. At baseline, in 153 individuals with complete data, DXA significantly over-estimated fat mass (Δ = 0.9, SD 2.1 kg, p<0.0001) and under-estimated lean mass (Δ = −1.0, SD 2.1 kg, p<0.0001). Multiple regression analysis showed that gender, puberty status, lean mass and fat mass were associated with the magnitude of the bias. In the longitudinal study of 51 individuals, the mean bias in change in fat or lean mass did not differ significantly from zero (FM: Δ = −0.02, p=0.9; LM: Δ = 0.04 p=0.8), however limits of agreement were wide (FM: ± 3.2 kg; LM ± 3.0 kg). The proportion of variance in the reference values explained by DXA was 76% for change in fat mass and 43% for change in lean mass. CONCLUSIONS: There are limitations to the accuracy of DXA using Lunar Prodigy for assessing body composition or changes therein in obese children. The causes of differential bias include variability in the magnitude of tissue masses, and stage of pubertal development. Further work is required to evaluate this scenario for other DXA models and manufacturers.