Reproducibility of bone mineral density measurement in daily practice

Background: Bone mineral density (BMD) measurements are frequently performed repeatedly for each patient. Subsequent BMD measurements allow reproducibility to be assessed. Objective: To examine the reproducibility of BMD by dual energy x ray absorptiometry (DXA) and to investigate the practical value of different measures of reproducibility in a group of postmenopausal women. Methods: Ninety five women, mean age 59.9 years, underwent two subsequent BMD measurements of spine and hip. Reproducibility was expressed as smallest detectable difference (SDD), coefficient of variation (CV), and intraclass correlation coefficient (ICC). Sources of variation were investigated by multilevel analysis. Results: The median interval between measurements was 0 days ( range 0 - 45). The mean difference (SD) between the measurements (g/cm(2)) was -0.001 (0.02) and -0.0004 (0.02) at L1- 4 and the total hip, respectively. At L1- 4 and the total hip, SDD (g/cm(2)) was +/- 0.05 and +/- 0.04 and CV (%) was 1.92 and 1.59, respectively. The ICC at spine and hip was 0.99. Conclusions: Reproducibility in the postmenopausal women studied was good. In a repeated DXA scan a BMD change exceeding 2root2CV (%), the least significant change (LSC), or the SDD should be regarded as significant. Use of the SDD is preferable to use of the CV and LSC (%) because of its independence from BMD and its expression in absolute units. Expressed as SDD, a BMD change of at least +/- 0.05 g/cm(2) at L1- 4 and +/- 0.04 g/cm(2) at the total hip should be considered significant.