Common Questions
How often should I repeat DXA?
For most women, the appropriate interval to repeat a DXA is 2 to 5 years; in a few circumstances, yearly may be indicated.
When to repeat a DXA depends on two factors:
- The anticipated rate of change in the patient's bone density
- The precision of measurement of the scanner
If the anticipated change is still within the "window of error" of the scanner, then the results are likely to be at best insignificant and at worst misleading.
In untreated patients. The average rate of loss of bone density among healthy postmenopausal women ranges from 1 to 2 percent per year. The reading on a scanner can vary 1 to 2 percent between repeated readings. Therefore to escape the error the expected change should be at least 2.77 percent--which is greater than the annual rate of loss in postmenopausal women not on any medications for osteoporosis. Therefore, repeat measurements should be every two years or even less frequently.
In treated patients. Similarly, bone density gains from treatment are about 1 to 2 percent per year. For patients receiving medications for osteoporosis or osteopenia, repeating DXA in 2 to 3 years is reasonable, 1 year at the earliest. Thus, even if one detects a significant drop in BMD in the first year of therapy, it may be premature to change your course of action. Recent data suggest that patients who show a significant loss of bone density after 1 year often show improvement after two, presumably because of regression to the mean (1).
One important exception is patients with secondary causes of osteoporosis. Patients on glucocorticoids, for example, can lose BMD at rates as high as 17 to 30 percent per year. Obviously, DXA scanning is useful at much shorter intervals for these patients.
| Precision Error of Technique | Statistically Meaningful Difference* | Assumed Rate of Bone Loss (per year) | Appropriate Follow-up Interval (in years) |
|---|---|---|---|
| 1% | +/- 2.77% | 1% | 3 years |
| 1% | +/- 2.77% | 3% | 1 year |
| 2% | +/- 5.54% | 1% | 5 to 6 years |
| 2% | +/- 5.54% | 3% | 2 years |
* This number reflects the amount two scans would have to differ for you to be sure a real change has occurred, with 95% confidence. Adapted from: Levis S, Altman R. Bone densitometry: Clinical considerations. Arthritis and Rheumatism. 1998; 41:577-587.
- Cummings SR, Palermo L, Browner W, et al. Monitoring osteoporosis therapy with bone densitometry: Misleading changes and regression to the mean. JAMA 2000; 283: 1318-1321.