The effect of weight change on DXA scans in a 2-year trial of etidronate therapy

Variation in soft tissue composition is a potential cause of error in dual X-ray absorptiometry (DXA) measurements of bone mineral density (BMD). We investigated the effect of patients' change of weight on DXA scans in 152 women enrolled in a 2-year trial of cyclical etidronate therapy. Scans of the spine, hip, and total body were performed at baseline, 1 and 2 years on a Hologic QDR-2000. The study was completed by 135 subjects (64 on etidronate, 71 on placebo). Results were expressed as the percentage change in BMD (spine, femoral neck, total body) or bone mineral content (BMC)(total body only) at 2 years. Total body scans were analyzed using the manufacturer's 'standard' and 'enhanced' algorithms. Analysis was performed using multivariate regression with percentage change in BMD or BMC as the dependent variable, and treatment group and percentage change in weight as the independent variables. Weight change varied between -14.4% and +16.7%. All DXA variables showed a statistically significant treatment effect. Standard total body BMD and BMC and enhanced total body BMC all showed a significant dependence on weight change (P < 0.01, P < 0.001 and P < 0.01, respectively). No effect of weight change was seen on spine, femoral neck, or enhanced total body BMD. In order to investigate the effects of weight on long-term precision, patients were allocated to two groups according to baseline body mass index (BMI < 25 and > 25 kg/m2, respectively). For femoral neck BMD the root mean square (RMS) residual percentage change was statistically significantly larger in the high BMI group (P < 0.05) but all other bone density variables showed no significant difference. With patients allocated to two groups according to their absolute percentage change in weight (< 5% and > 5%, respectively) the RMS residual percentage changes in the bone density variables were statistically significantly larger in the large weight change group for femoral neck BMD (P < 0.05) and for standard and enhanced total body BMC (P < 0.01 and P < 0.05, respectively). With the exception of the standard total body algorithm, weight change in a longitudinal study of postmenopausal women was not found to cause systematic errors in the results of DXA studies but may adversely affect precision.