Reproducibility of DXA: Potential Impact on Serial Measurements and Misclassification of Osteoporosis

Abstract:

The reproducibility of bone mineral density (BMD) mesurements by dual-energy X-ray absorptiometry (DXA), based on 12 successive monthly determinations, was assessed in a group of 24 subjects (23 postmenopausal women, 1 man) using six trained operators. The variability (S ² _A) was calculated from both duplicate operator measurements and the standard error of estimate from nonparametric regression of the individual subject series. Robust estimates of S _A from the 90th percentile of the sampling distribution of variances were calculated for the spine (25 mg/cm²), femur neck (20 mg/cm²) and total femur (15.5 mg/cm²) using the bootstrap technique. The critical difference for a significant decrease (p=0.05) at the spine, femoral neck and total femur was estimated at 57, 46 and 36 mg/cm² respectively. Estimation of S ² _A allowed calculation of the probability that the true BMD, for an observed BMD near the osteoporosis diagnostic threshold (T-score <−2.5), is not misclassified. Analysis of covariance established a significant operator–subject interaction at all sites, but only the total femur was associated with a significant difference between operators. The percentage of body fat was a significant covariate for the spine and total femur regions. ANOVA showed that the greater proportion of variance was instrument-related. The limitations of DXA as an analytical method are discussed.