Precision and accuracy of measuring changes in bone mineral density by dual-energy X-ray absorptiometry
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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.
Long-term precision and accuracy of BMD measurements need review.
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 (Δ) in BA, BMD and BMC.
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 ΔBA and ΔBMC were found. These led to an underestimate of ΔBMD in spine and hip by 25%.
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.
KeywordsAccuracy BMD DXA Precision
We are grateful to Professor Stuart Ralston for valuable discussions, to Professor Richard Anderson and Dr David Cameron for permission to use results from their clinical trial, and to Colin Ferrington and Carol Millar for skilled technical assistance.
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