DUAL-ENERGY X-RAY ABSORPTIOMETRY - FAT ESTIMATION ERRORS DUE TO VARIATION IN SOFT-TISSUE HYDRATION

Dual-energy X-ray absorptiometry (DXA) is rapidly gaining acceptance a s a reference method for analyzing body composition. An important and unresolved concern is whether and to what extent variation in soft tis sue hydration causes errors in DXA fat estimates. The present study ai m was to develop and validate a DXA physical hydration model, and then to apply this model by simulating errors arising from hypothetical ov erhydration states. The DXA physical hydration model was developed by first linking biological substance elemental content with photon atten uation. The validated physical model was next extended to describe pho ton attenuation changes anticipated when predefined amounts of two kno wn composition components are mined, as would occur when overhydration develops. Two overhydration models were developed in the last phase o f study, formulated on validated physical models, and error was simula ted for fluid surfeit states. Results indicate that systematic errors in DXA percent fat arise with added fluids when fractional masses are varied as a percentage of combined fluid + soft tissue mass. Three ind ependent determinants of error magnitude were established: elemental c ontent of overhydration fluid, fraction of combined fluid + soft tissu e as overhydration fluid, and initial soft tissue composition. Small b ut systematic and predictable errors in DXA soft tissue composition an alysis thus: can arise with fluid balance changes.