Precision and accuracy of measuring changes in bone mineral density by dual-energy X-ray absorptiometry.
Long-term precision of two Hologic DXA scanners was derived from repeated clinical measurements. With typical subjects, the long-term coefficients of variation were about twice the short-term. The accuracy of the measurement of changes was compromised by anomalies, but this did not seriously affect clinical conclusions.


BACKGROUND
Long-term precision and accuracy of BMD measurements need review.


METHODS
Long-term precision was examined by selecting, from Hologic databases, subjects who had had four scans over a period of 2 to 5 years and was calculated from the SEE of a regression of BMD against time. Accuracy was assessed from relationships between changes (delta) in BA, BMD and BMC.


RESULTS
For one group of subjects, the long-term precision was 2.4% for lumbar spine, 2.3% for total hip and 2.7% for femoral neck when expressed as CVs. These values were nearly twice the short-term CVs of 1.3% for spine, 1.2% for total hip and 1.4% for femoral neck. For another group, a negative exponential regression gave a better fit, leading to CVs of 1.3% for the spine, 1.4% for total hip and 2.1% for femoral neck. Significant correlations between deltaBA and deltaBMC were found. These led to an underestimate of deltaBMD in spine and hip by 25%.


CONCLUSIONS
The poorer long-term precision for typical patients should be borne in mind in monitoring progress. The underestimate of changes could account for only a part of the underestimation by BMD measurements of the anti-fracture effects of anti-resorptive drugs.
Tothill
P
P
Department of Medical Physics, Western General Hospital, Edinburgh EH4 2XU, UK. peter.tothill@ed.ac.uk
Hannan
W J
WJ
D015502:Absorptiometry, Photon / Q000379:methods / Q000592:standards; D000293:Adolescent; D000328:Adult; D000368:Aged; D015519:Bone Density; D005260:Female; D005272:Femur Neck / Q000503:physiopathology; D005500:Follow-Up Studies; D006621:Hip Joint / Q000503:physiopathology; D006801:Humans; D008159:Lumbar Vertebrae / Q000503:physiopathology; D008875:Middle Aged; D010024:Osteoporosis / Q000175:diagnosis* / Q000503:physiopathology; D019047:Phantoms, Imaging; D015203:Reproducibility of Results