Body-composition measurement in 9-11-y-old children by dual-energy X-ray absorptiometry, skinfold-thickness measurements, and bioimpedance analysis.

We compared, in 9-11-y-old children (n=43), three measures of body composition: dual-energy X-ray absorptiometry (DXA), skinfold thickness, and bioimpedance analysis (BIA). The intraclass correlation coefficient (ICC), Bland-Altman procedure, and Spearman rank correlation were used to determine test-retest reliabilities of the three methods and to compare methods. For DXA measurements, the rank correlation between fat-free soft tissue and fat-free mass (FFM) was > 0.99, indicating that bone mineral content did not provide independent information. Thus, subsequent analyses used the two-compartment model (ie, fat mass and FFM) for all three techniques, focusing especially on values for percentage of fat. The test-retest reliabilities for all methods were high (ICCs > 0.994 and no significant differences between trials 1 and 2). The range of individual differences from trial 1 to trial 2 and Bland-Altman limits of agreement suggested that the reliability was greatest for DXA, followed by BIA and skinfold-thickness measurement. The percentage of fat values for the three methods were highly intercorrelated (all Spearman r values > 0.83). However, there was a systematic tendency (P < 0.01) for DXA values (mean: 23.98) to be higher than those derived from skinfold-thickness measurements (mean: 21.05) and BIA (mean: 21.52). The variance in percentage of fat values for BIA was significantly smaller than that for the other two techniques. These findings, along with rather large limits of agreement derived from the Bland-Altman procedure, suggest that the methods should not be used interchangeably.