Measurement error of DXA: interpretation of fat and lean mass changes in obese and non-obese children.

Information on reproducibility of dual-energy X-ray absorptiometry (DXA) measurements is essential because DXA is frequently used by clinicians and researchers to assess body composition changes. We estimated measurement error and absolute and relative smallest detectable differences (SDDs) for fat, lean, and bone mass in children. The SDD is the change necessary to be confident that the change is not a consequence of measurement error. Duplicate whole body DXA (Hologic QDR 4500A, Hologic Inc., Waltham, MA) scans were obtained on 32 obese and 34 non-obese children ages 6-19 yr. Absolute (kg) and relative (coefficient of variation) measurement error and SDD were calculated. Absolute SDDs for fat and lean were higher for obese (1.39 and 1.30 kg, respectively) than for non-obese children (0.42 and 0.47 kg, respectively). The %SDD for fat was lower for obese (3.58%) than non-obese children (5.24%), but for lean the %SDD was higher for obese (2.60%) than non-obese children (1.32%). The SDDs for bone mass were similar for obese and non-obese children. An obese child must lose or gain more absolute fat and lean mass than a non-obese child to be confident that the change is not a reflection of measurement error. Overall, SDD values for fat, lean, and bone mass are low.