Background: Dual-energy X-ray absorptiometry (DXA) has been used extensively to measure body composition, but has been validated by comparison to chemical analysis on relatively few occasions. Moreover, these previous validation studies have ground up entire carcasses prior to chemical analysis, thus potentially obscuring sources of error in the DXA analysis.
Objective: The purpose of this study was to validate DXA by comparison to chemical analysis in dogs and cats, performing chemical analysis on dissected rather than ground carcasses to reveal sources of discrepancy between the two methods.
Design: Sixteen animals (10 cats and 6 dogs weighing between 1.8 and 22.1 kg) were scanned by DXA post-mortem using a Hologic QDR-1000 W pencil beam machine and then dissected into 22 separate components. Individual tissues were dried and then sub-sampled for analysis of fat content by Soxholet extraction, or ashing in a muffle furnace. Body composition by DXA was compared to body composition by chemical analysis and discrepancies between the two correlated with chemical composition of individual tissues. We also explored the capability of the machine to establish the fat contents of mixtures of ground beef, lard and water.
Results: DXA estimates were strongly correlated with estimates derived from chemical analysis: total body mass (r=1.00), lean tissue mass (r=0.999), body water content (r=0.992) and fat mass (r=0.982). Across individuals the absolute and percentage discrepancies were also small: total body mass (13.2 g, 1.02%), lean tissue mass (119.4 g, 2.64%), water content (101 g, 1.57%) and fat content (28.5 g, 2.04%), where the percentage error is expressed relative to the average mass of that component across all individuals. Although on average DXA compared very well to chemical analysis, individual errors were much greater. Individual errors in the lean tissue and fat tissue components were strongly correlated with the fat content of skeletal muscle and the lean content of mesenteric fat. The error in the DXA estimate of total fat content was related to skeletal muscle hydration. Experimental studies using mixtures of lean ground beef, water and lard indicated that tissue hydration may have important effects on the perception of tissue fat content by DXA. Bone mineral content by DXA was approximately 30% lower than whole body ash content but only 7.7% lower than ash content of the bones.
Conclusions: On average pencil beam DXA analysis using the Hologic QDR-1000 W machine provides an accurate estimate of body composition in subjects weighing between 1.8 and 22.1 kg. Individual discrepancies, however, can be large and appear to be related to lean tissue hydration.