Figure 11. Quality assurance plots from three hypothetical systems with mean 1.000, standard deviation 0.005 (coefficient of variation 0.5%). The top plot indicates a stable machine, the middle plot indicates an abrupt shift in baseline (regression lines plotted to the data prior to and after the shift) and the bottom plot indicates a continuous drift in baseline (regression line plotted to all time points). The latter is particularly insidious and would be difficult to identify by visual inspection of the data alone.

menopausal Caucasian females, therefore caution must be exercised in extrapolating these to other groups (Table 4).

Predicting Fractures from Bone Density Measurements

As already noted, the mechanical strength of excised bone is strongly related to the amount of bone mineral. Many prospective studies have now shown that bone measurements will predict clinical fractures (Fig. 13). Although bone density is on average significantly lower in fracture patients than in non-fracture patients, there is considerable overlap between the two groups. Risk of fracture shows a continuous gradient relationship with bone density: there is no "fracture threshold". Results are usually stated in terms of relative risk (RR) of fracture per standard deviation (SD) change in bone density. It appears that any measured site provides fracture risk information about other sites, though the best measure for characterizing hip fracture risk is a measure of the proximal femur (with RR 2.6 per SD change in bone density). It will be recalled that a unit change in SD is the same as a unit change in the Z-score

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