Bone mineral density testing: Is a T score enough to determine the screening interval?
ABSTRACTTo find the rational intervals for bone mineral density screening, Gourlay et al (N Engl J Med 2012; 366:225–233) used T scores to calculate the time required for women age 67 and older with normal bone mineral density or osteopenia to progress to osteoporosis. They estimated that the screening interval for women with normal bone mineral density or mild osteopenia (T score –1.49 or higher) could be as long as 15 years. However, the investigators focused mainly on T scores and when these scores reached –2.5. In our opinion, the testing interval should be guided by an assessment of clinical risk factors and not just baseline T scores.
KEY POINTS
- The criteria for who should undergo bone mineral density measurement are well established, but data on repeat testing are scarce.
- Gourlay et al concluded that age and T scores are the key predictive factors in determining the bone mineral density testing interval, while clinical risk factors such as fracture after age 50, current smoking, previous or current use of glucocorticoids, and self-reported rheumatoid arthritis are not.
- The Fracture Risk Assessment tool (FRAX) is a useful clinical tool that calculates an individual’s 10-year risk of fracture. It is available at www.shef.ac.uk/FRAX
Some members of the public may have noticed the conclusions of a recent study1 that said that if an older postmenopausal woman has her bone mineral density measured to screen for osteoporosis and has a normal or only mildly low result, she does not need to come back for another measurement for approximately 15 years.
We believe this interpretation of the study’s findings is overly simplistic and may have the unfortunate result of causing some people to neglect their bone health. Moreover, the study looked mainly at baseline T scores as the determinant of the subsequent screening interval. However, clinicians must carefully consider a variety of clinical risk factors when deciding how often to obtain bone mineral density measurements. The ultimate goal is to not miss the window of opportunity for early detection and treatment when it would matter the most (ie, before fractures develop).
Here, we will review this recent study, its findings, and its implications.
OSTEOPOROSIS POSES AN ENORMOUS PUBLIC HEALTH PROBLEM
If we consider only the hip, an estimated 10 million people in the United States have osteoporosis (T score ≤ −2.5 or a preexisting fragility fracture), and 33.6 million have osteopenia (T score −1.01 to −2.49).2 The number of people with osteopenia can be assumed to be much higher if other skeletal sites are considered.
By increasing the risk of fragility fractures, osteoporosis poses an enormous public health problem. The surgeon general’s report points out that one of every two white women over age 50 will experience an osteoporosis-related fracture in her lifetime.3 Of all osteoporosis-related fractures, those of the hip carry the worse clinical outcome. Approximately one in five elderly people who experience an osteoporosis-related hip fracture need long-term nursing home care, and as many as 20% die within 1 year.3
In recognition of the burden of osteoporosis, the US Preventive Services Task Force (USPSTF)4 and other scientific bodies2,3 recommend an initial bone mineral density test for all women age 65 and older. Dual-energy x-ray absorptiometry (DXA) is considered the gold standard for bone mineral density testing. Although the patient population that should receive an initial bone mineral density test has been clearly identified (see below), guidelines on the optimal frequency of testing do not exist, as data have been lacking. Recognizing this knowledge gap, Gourlay et al1 attempted to answer the question of how often elderly postmenopausal women should be retested.
WHEN DO 10% OF ELDERLY POSTMENOPAUSAL WOMEN REACH A T SCORE OF −2.5?
Gourlay et al1 analyzed data from 4,957 women in the Study of Osteoporotic Fractures. These women were predominantly white, were at least 67 years old and ambulatory, and had normal bone mineral density or osteopenia and no history of hip or clinical vertebral fracture at baseline. They had been recruited between 1986 and 1988 at sites in Baltimore, MD, Minneapolis, MN, the Monongahela Valley near Pittsburgh, PA, and Portland, OR.
DXA of the hip had been performed at baseline and at multiple times thereafter. The average follow-up time was 8 years.
The primary outcome measured was how long it took for 10% of the patients to reach a T score of −2.5 or less at the femoral neck or total hip as they progressed from having normal bone mineral density to osteoporosis or from osteopenia to osteoporosis and before they developed a fracture or needed treatment for osteoporosis.
Clinical risk factors such as age, body mass index, estrogen use at baseline, fracture after age 50, current smoking, current or past use of glucocorticoids, and self-reported rheumatoid arthritis were included as covariates in time-to-event analyses.
ANSWER: 16.8 YEARS (IF NORMAL AT BASELINE)
The authors estimated that 10% of women would make the transition to osteoporosis before having a hip or clinical vertebral fracture in the following intervals:
- 16.8 years in women whose bone mineral density was normal at baseline (T score at femoral neck and total hip of −1.00 or higher)
- 17.3 years in women who had mild osteopenia at baseline (T score −1.01 to −1.49)
- 4.7 years in women with moderate osteopenia at baseline (T score −1.5 to −1.99)
- 1.1 years in women with advanced osteopenia at baseline (T score −2.00 to −2.49).
The authors also found that body mass index and current estrogen use were the only clinical risk factors that influenced these intervals; other clinical factors such as a fracture after age 50, current smoking, previous or current use of oral glucocorticoids, and self-reported rheumatoid arthritis did not.
They concluded that osteoporosis would develop in fewer than 10% of women if the rescreening interval was lengthened to 15 years for women with normal density or mild osteopenia, 5 years for women with moderate osteopenia, and 1 year for women with advanced osteopenia.
