124 Chapter 6 Figure 2. Bland-Altman plot of fat mass percentage measured by DXA compared with ADP ADP air displacement plethysmography, DXA dual-energy X-ray Average of fat mass measured with DXA and ADP is depicted on the x-axis and the difference between the fat mass percentage measured with DXA and ADP is depicted on the y-axis. Mean difference: 4.5 ± 4.7%, lower limit of agreement: - 4.8%, upper limit of agreement: 13.7%, maximum allowed difference: 2% DXA compared to skinfolds The difference between fat mass estimated through the model of Schmelzle and Fusch, and the fat mass measured with DXA exceeded the limits of agreement. The mean difference was 272 + 240 g, with a lower limit of agreement of -742 g and an upper limit of agreement of 199 g. Fat mass percentage derived from skinfolds did not agree with fat mass percentage measured with DXA and showed a proportional bias with a larger difference in fat mass percentage with increasing mean fat mass percentage. (Figure 3) Predictive model for fat mass measured with DXA Within our cohort, fat mass, measured with DXA, could be estimated with gestational age, waist circumference, length and the ∑SFT: Fat mass (g) = -4649.1 + 23.5*∑SFT + 64.4*length + 77.6*waist circumference – 33.7*gestational age (∑SFT in mm, length and waist circumference in cm and gestational age in weeks). These factors explained 89% of the variance (R2 = 0.893, S.E. of the estimate 146 g, , p < 0.001). In addition, 75% of the variance in fat mass percentage, measured with DXA, could be explained by waist circumference, head circumference and the ∑SFT (R2 = 0.753, S.E. of the estimate 3.5 % 4, p < 0.001).
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