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precision as «1.0% for modern DEXA instruments. This probably underestimates the error seen in non-research, clinical populations. In routine clinical settings the following reproducibilities (%CV) have been reported: lumbar spine 1.8%, femoral neck 3.5%, total hip 2.5%.

Quality Control

A formal quality assurance program is an essential component of a bone density program. In the case of DEXA, this should minimally consist of a daily calibration check (usually with an anthropomorphic spine phantom) which is compared with predefined tolerance limits. The cumulative data is periodically analyzed to look for subtle changes or drifts in performance signaling the need for corrective action (Fig. 11). Calibration differences between otherwise identical machines are common. Such

Figure 10. Paget's disease of the right hip. The bone expansion and sclerosis give a bone density measurement of the femoral neck that is well above normal (1.314 g/ cm2 with T-score +2.78) while the left hip is actually in the osteoporotic range (0.634 g/cm2 with T-score -2.85).

differences are usually small (1-2%) but on occasion can be clinically significant (exceeding instrument reproducibility). Therefore, measurements from different machines are very difficult to compare, and whenever possible follow-up examinations should be performed on the same machine.

Clinical Role of Bone densitometry

T-Score and Z-Score

Absolute measurements of bone density are of little value since they are determined by the site of measurement, calibration used by the equipment manufacturer, and even the particular instrument. Since bone density follows a bell-shaped (Gaussian) distribution, measurements are conventionally described in terms of the number of

Table 2. Interval in years between two bone density measurements required to determine significant bone loss (P<0.05) in an individual patient

Rate of bone loss (percent/year)

Table 2. Interval in years between two bone density measurements required to determine significant bone loss (P<0.05) in an individual patient

Rate of bone loss (percent/year)

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