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
The study did not address asymptomatic vertebral fractures and lumbar spine density
Gourlay et al1 did not take into account asymptomatic spinal fractures; they used only clinical vertebral fractures in their risk estimates of spinal fractures. Ascertainment of morphometric spinal fractures may be methodologically challenging, but if the study had included these fractures, the outcomes and conclusions could have been very different.
Vertebral fractures are present in as many as 14% to 33% of postmenopausal women17 and indicate osteoporosis (regardless of the bone mineral density). Moreover, most vertebral fractures are clinically silent and escape detection, and approximately only one in three radiographically defined vertebral fractures is reported clinically.18,19 Given the prevalence of these fractures, we and others10 have noted that the results of the Gourlay study may be biased toward longer screening intervals because they did not account for morphometric vertebral fractures.
Gourlay et al used T scores only of the femoral neck and total hip and not those of the lumbar spine. Some studies have found that hip measurements may be superior to spine measurements for overall osteoporotic fracture prediction.20,21 However, lumbar spine bone mineral density is predictive of fracture at other skeletal sites,22,23 is a widely accepted skeletal site measurement, and is used to diagnose osteoporosis. Moreover, the lumbar spine T score can be −2.5 or higher even if the total hip or femoral neck T score is lower than −2.5.
More fractures occur in people with osteopenia than with osteoporosis
Osteoporosis imparts a much higher risk of fracture than does osteopenia. However, if one recognizes the much greater prevalence of osteopenia (33.6 million people) compared with osteoporosis (10 million),2 it is not hard to appreciate that the number of fractures is higher in the osteopenic group than in those with osteoporosis based on T scores. Siris et al24 point out that at least half of osteoporotic fractures are in patients with osteopenia, who comprise a larger segment of the population than those with osteoporosis.
Some clinical trials have shown that bisphosphonates are not effective in preventing clinical fractures in women who do not have osteoporosis.25,26 However, clinicians must recognize that while bisphosphonates may not be as effective in preventing fractures in the osteopenic group with no other clinical risk factors, the presence of multiple clinical risk factors incrementally increases the fracture risk (which can be assessed via FRAX) and may require starting drug therapy earlier.
Women with vertebral fractures are considered to have clinical osteoporosis even if they have T scores in the osteopenic range, and must be considered for drug therapy.
The public health burden of fractures will not decrease unless individuals with low bone mineral density who are at an increased risk of fracture are identified and treated.24
Is DXA testing overused or underused? does it decrease the rate of fractures?
The study of Gourlay et al1 captured a lot of media attention, with many newspapers and blogs claiming that women may be getting tested too often.27,28 However, in reality, this test is highly underutilized. The 2011 Healthcare Effectiveness Data and Information Set report noted that 71.0% of women in Medicare health maintenance organizations and 75.0% of women in Medicare preferred provider organizations ever had a bone mineral density test for osteoporosis.29 While these numbers may not appear to be too far from the target, they are a poor gauge of DXA use as they include all types of bone mineral density tests in a woman’s lifetime, including even heel tests at health fairs.
Central DXA is used far less than one might expect. King and Fiorentino, in a recent analysis, showed that only about 14% of fee-for-service Medicare beneficiaries 65 years and older had one or more DXA tests in 2010.30 DXA retesting also does not seem to be an issue, with only 1 in 10 elderly women reporting having had a repeat test at 2-year intervals, and fewer than 1 in 100 tested reported testing more frequently.30
