Dual-energy X-ray absorptiometry: analysis of pediatric fat estimate errors due to tissue hydration effects

Dual-energy X-ray absorptiometry (DXA) percent (%) fat estimates may be ina ccurate in young children, who typically have high tissue hydration levels. This study was designed to provide a comprehensive analysis of pediatric t issue hydration effects on DXA %fat estimates. Phase 1 was experimental and included three in vitro studies to establish the physical basis of DXA %fa t-estimation models. Phase 2 extended phase I models and consisted of theor etical calculations to estimate the %fat errors emanating from previously r eported pediatric hydration effects. Phase I experiments supported the two- compartment DXA soft, tissue model and established that pixel ratio of low to high energy (R values) are a predictable function of tissue elemental co ntent. In phase 2, modeling of reference body composition values from birth to age 120 mo revealed that %fat errors will arise if a "constant" adult l ean soft tissue R value is applied to the pediatric population; the maximum %fat error, similar to0.8%, would be present at birth. High tissue hydrati on, as observed in infants and young children, leads to errors in DXA %fat estimates. The magnitude of these errors based on theoretical calculations is small and may not be of clinical or research significance.