Evaluation of bioelectrical impedance analysis in measuring body fat in 6-to-12-year-old boys compared with air displacement plethysmography.

Air displacement plethysmography (ADP) has been considered as the 'standard' method to determine body fat in children due to superior validity and reliability compared with bioelectrical impedance analysis (BIA). However, ADP and BIA are often used interchangeably despite few studies comparing measures of percentage body fat by ADP (%FM ADP ) with BIA (%FM BIA ) in children with and without obesity. The objective of this study was to measure concurrent validity and reliability of %FM ADP and %FM BIA in 6-to-12-year-old boys with and without obesity. Seventy-one boys (twenty-five with obesity) underwent body composition assessment. Ten boys participated in intra-day reliability analysis. %FM ADP was estimated by Bodpod using sex- and age-specific equations of body density. %FM BIA was estimated by a multi-frequency, hand-to-foot device using child-specific equations based on impedance. Validity was assessed by t tests, correlation coefficients and limits of agreement (LoA); and reliability by technical error of measurement (TEM) and intraclass correlation coefficients (ICC). Compared with %FM ADP , %FM BIA was significantly underestimated in the cohort (-3·4 ± 5·6 %; effect size = 0·42) and in both boys with obesity (-5·2 ± 5·5 %; ES = 0·90) and without obesity (-2·4 ± 5·5 %; ES = 0·52). A strong, significant positive correlation was found between %FM ADP and %FM BIA ( r = 0·80). Across the cohort, LoA were 22·3 %, and no proportional bias was detected. For reliability, TEM were 0·65 % and 0·55 %, and ICC were 0·93 and 0·95 for %FM BIA and %FM ADP , respectively. Whilst both %FM ADP and %FM BIA are highly reliable methods, considerable differences indicated that the devices cannot be used interchangeably in boys age 6-to-12 years.