Bone mineral density testing

Clinical question

Once we have initiated bisphosphonate therapy, how often should we check bone mineral density (BMD)?

Evidence

Secondary analysis of the Fracture Intervention Trial:

• 6459 patients randomized to receive alendronate or placebo, with annual BMD testing for 3 years.1

  • -Mean increase in hip BMD of 0.030 g/cm² in the alendronate group compared with a mean decrease of 0.012 g/cm² with placebo.

  • -Individuals’ BMD readings were more variable than readings between people.

  • -Alendronate increased BMD 0.013 g/cm² per year, but individuals’ readings varied by a similar amount (SD 0.012 g/cm²).

  • -Alendronate resulted in “sufficient” (≥ 0.019 g/cm²) increases in hip BMD for 97.5% of patients after 3 years.

• The Fracture Intervention Trial also demonstrated that women with decreased BMD2 who took alendronate still had reductions in fracture risk.

Context

• Dual-energy x-ray absorptiometry BMD measurement precision has important limitations.

  • -535 patients scanned twice over 2 to 4 weeks demonstrated variability in BMD at the hip of 2.4% (trochanter) to 5% (Ward’s triangle).3

  • -Precision of measurements declines with decreasing BMD.4

• Canadian 2010 clinical practice guidelines recommend repeating BMD 1 to 3 years after initiating therapy.5

  • -However, average rate of bone loss in postmenopausal women is 0.5% to 2.0% per year, while most treatments increased BMD 1% to 6% over 3 years.6

  • -Given these very small changes, only a very precise test will detect short-term changes.

• Measurement of BMD is too imprecise to reliably discern the small annual changes resulting from therapy.

Bottom line

Repeating BMD testing within 3 years of starting treatment with bisphosphonates is unnecessary and potentially confusing.1 By far most patients taking bisphosphonates will have adequate increases in BMD after 3 years and will have a reduced fracture risk regardless of BMD changes.

Implementation

Osteoporosis is the risk factor and surrogate marker for the clinically meaningful end point of symptomatic fracture. To estimate baseline fracture risk, Canadian guidelines5 recommend tools like CAROC,7 based on age, sex, and T score for the femoral neck, or FRAX, a complex online tool.8 Evidence suggests that more complex prediction models such as FRAX do not significantly improve fracture prediction over simple tools based on age and BMD9 or BMD alone in early postmenopausal women.10 If your BMD reports do not include an assessment of fracture risk, a simple table or graph of CAROC can be found at the Osteoporosis Canada website.7 Once bisphosphonate therapy has been initiated, repeat BMD measurements are not routinely necessary before 3 years’ time.