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standard deviations (SD) that a value deviates from the population mean. Age-related changes in bone density, described earlier, must be taken into account. The Z-score refers to the number of SD above or below the mean for an age-matched population. The T-score refers to the number of SD above or below the mean for a young adult population. It is not enough for a bone density instrument to provide accurate and reproducible measurements. To interpret a patient value there must be valid normal data from a large reference population that is patient-appropriate (age, gender and ethnicity).

In interpreting an individual patient's test results, the following question is confronted: Should you compare the patient with someone of the same age or with a young adult? The former masks the increasing prevalence of osteoporosis with advancing age, while judging an 80-year-old against the same standard used in a 30-year-old seems unreasonable. In reality, both approaches have merit and are complementary (Fig. 12). An age-adjusted measurement indicates whether the individual is average for their age and, if not, how markedly they deviate from the expected value. On the other hand, bone strength depends upon bone mass and not the age of the subject, therefore predictions in terms of fracture risk are best based upon comparison with an absolute standard (young adult).

Diagnosis of Osteoporosis from Bone Density Measurements

The World Health Organization (WHO) formulated diagnostic ranges for osteoporosis based upon T-score. These ranges were originally intended to be used epidemiologically, but have subsequently been applied to individuals. The data reviewed for th ese recommendations was almost exclusively derived from post-

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.

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