Precision of measurement and body size in whole-body air-displacement plethysmography

OBJECTIVE: To investigate methodological and biological precision for air-displacement plethysmography (ADP) across a wide range of body size. DESIGN: Repeated measurements of body volume (BV) and body weight (WT), and derived estimates of density (BD) and indices of fat mass (FM) and fat-free mass (FFM). SUBJECTS: Sixteen men, aged 22-48 y; 12 women, aged 24-42 y; 13 boys, aged 5-14 y; 17 girls, aged 5-16 y. MEASUREMENTS: BV and WT were measured using the Bodpod ADP system from which estimates of ED, FM and FFM were derived. FM and FFM were further adjusted for height to give fat mass index (FMI) and fat-free mass index (FFMI). RESULTS: ADP is very precise for measuring both BV and ED (between 0.16 and 0.44% of the mean). After removing two outliers from the database, and converting ED to body composition, precision of FMI was < 6% in adults and within 8% in children, while precision of FFMI was within 1.5% for both age groups. CONCLUSION: ADP shows good precision for BV and ED across a wide range of body size, subject to biological artefacts. If aberrant values can be identified and rejected, precision of body composition is also good. Aberrant values can be identified by using pairs of ADP procedures, allowing the rejection of data where successive ED values differed by > 0.007 kg/l. Precision of FMI obtained using pairs of procedures improves to < 4.5% in adults and < 5.5% in children.