Bone mass measurement by DXA: influence of analysis procedures and interunit variation

Interunit variability among bone densitometers is due to different factors, including different calibration procedures and algorithms and variability in photon source energies and/or intensities. Other factors, such as the choice of scan parameters or the analysis procedures, can also introduce variability. The new generation of dual-energy x-ray absorptiometry (DXA) has partially improved this situation. The aim of this study was to investigate the operator-dependent analysis procedures that can affect scan results and to evaluate the phantom and in vivo interunit variation of some DXA instruments. Four DXA instruments (QDR 1000 and 1000/W, Hologic, Inc.) were used. Potential sources of variability in the analysis procedures of anteroposterior lumbar spine and hip scans were considered: in most cases these procedures significantly influenced scan results. On lumbar spine, an enlargement of the scan window of less than 3 cm was responsible for an average increase in bone mineral density (BMD) of about 3%. On the hip, lowering the scan window by about 1 cm accounted for an increase in the whole-segment BMD of about 4%. After standardization of analysis procedures, interunit and intraunit coefficients of variation and percentage differences among instruments were less than 1% for all the parameters considered (area and bone mineral content and density) with both an anatomic and a geometric phantom, and in nine subjects scanned by two different devices the percentage difference in BMD was greater than 2%. This study shows that present interunit variability allows comparisons among laboratories, but only if highly standardized analysis procedures are used.