Bone mineral density testing: Is a T score enough to determine the screening interval?

WHAT DOES THIS MEAN FOR THE PRACTICING CLINICIAN?

Who needs an initial DXA test according to current guidelines?

The USPSTF,⁴ the National Osteoporosis Foundation (NOF),⁵ the International Society for Clinical Densitometry (ISCD),⁶ and the American Association of Clinical Endocrinologists (AACE)⁷ propose that the following groups should undergo DXA:

• All women age 65 and older
• All postmenopausal women who have had a fragility fracture or who have one or more risk factors for osteoporosis (height loss, body mass index < 20 kg/m², family history of osteoporosis, active smoking, excessive alcohol consumption)
• Adults who have a condition (eg, rheumatoid arthritis) or are taking a medication (eg, glucocorticoids in a daily dose ≥ 5 mg of prednisone or its equivalent for ≥ 3 months) associated with low bone mass or bone loss
• Anyone being considered for drug therapy for osteoporosis, discontinuing therapy for osteoporosis (including estrogen), or being treated for osteoporosis, to monitor the effect of treatment.

Assessing fracture risk. Although clinicians have traditionally relied on bone mineral density obtained by DXA to estimate fracture risk, the World Health Organization has developed a computer-based algorithm that calculates an individual’s 10-year fracture probability from easily obtained clinical risk factors with or without adding femoral-neck bone mineral density. The Fracture Risk Assessment tool, or FRAX, has attracted intense interest since its introduction in 2007 and has been endorsed by the USPSTF⁴ and by other scientific societies, including the NOF⁵ and the ISCD.⁸ In fact, the most recent USPSTF guidelines,⁴ which recommend screening all women age 65 and older, call for using FRAX to identify younger women at higher risk of fracture.

According to FRAX, a 65-year-old white woman who has no risk factors has a 9.3% chance of developing a major osteoporotic fracture in the next 10 years. And if a younger woman (between the ages of 50 and 64) has a fracture risk as high or higher than a 65-year-old white woman who has no risk factors, then she too should be screened by DXA.

The FRAX calculator is available online at www.shef.ac.uk/FRAX.

What are the current recommendations about follow-up DXA testing?

In eligible patients, the Centers for Medicare and Medicaid Services will pay for a DXA scan every 2 years. This interval is based on the concept that in an otherwise healthy person, it takes a minimum of 2 years to see a significant change in bone mineral density that can be attributed to a biological change in the bone and not just chance. The USPSTF⁴ and scientific societies such as the NOF⁵ generally agree with the Medicare guidelines of retesting every 2 years but recognize certain clinical situations that may warrant more frequent retesting (see below).

But the real question is how long the DXA screening interval can be extended so that meaningful information can still be obtained to help make management decisions and before a complication such as a fracture occurs. While there is convincing evidence to support the recommendations for an initial DXA test, data to answer the question of how long the resting interval should be are lacking.

Before the study by Gourlay et al,¹ the only data on repeat DXA came from work by Hillier et al.⁹ But those investigators asked a different question. They were interested in how well repeated measurements predicted fractures. They used the same population that Gourlay et al did but evaluated fractures, not T scores. They concluded that in healthy, adult postmenopausal women, repeating the bone mineral density measurement up to 8 years later adds little value to initial measurement for predicting incident fractures.

Clinical factors also count

The T score should not be the only major factor determining the interval for bone mineral density testing in elderly women; clinical risk factors also should be kept in mind.

Gourlay et al concluded that age and T scores are the key predictive factors in determining the bone mineral density testing interval in elderly, postmenopausal women for screening purposes.¹ In their statistical model, clinical risk factors such as fracture after age 50, current smoking, previous or current use of glucocorticoids, and self-reported rheumatoid arthritis did not influence the testing interval. They say that clinicians should not feel compelled to shorten the testing interval when these risk factors are present.

Readers may take this to mean that if these results were strictly applied to a 70-year-old white woman receiving oral glucocorticoids for rheumatoid arthritis and who has a baseline T score of −1.45, then her next test may be postponed by 15 years (given that both these factors did not influence the testing interval). Readers may also conclude that if this patient’s T score were −1.51, then her screening interval would be 5 years and not 15 years.

However, Gourlay et al say¹ that clinicians can choose to shorten the testing interval if there is evidence of decreased activity or mobility, weight loss, or other risk factors not considered in their analysis.

Soon after this study1 was published, Lewiecki et al¹⁰ and others^11–13 published critical commentaries addressing controversial issues surrounding the study. They highlighted the importance of considering clinical risk factors for fracture in addition to the femoral neck and total hip T scores. In response to these comments, Gourlay et al clarified that their results were not generalizable to patients with secondary osteoporosis, such as those taking glucocorticoids or those who have rheumatic diseases.¹⁴

Readers should keep in mind that clinical risk factors make independent contributions to fracture risk (Figure 1).¹⁵

Readers should also recognize the following groups in whom the results of the study by Gourlay et al are not applicable since they were not included in their study:

• Men
• Women other than white women
• Women already diagnosed with osteoporosis and on bisphosphonates or any other osteoporosis treatment (except for estrogen). The findings also do not apply to:
• Patients who experience a significant decline in health status or who develop new clinical conditions (such as hyperparathyroidism, paraproteinemias, or type 2 diabetes) or who use medications such as glucocorticoids that cause rapid bone loss. Changes in clinical situations such as these may necessitate more frequent bone mineral density testing in spite of a “good” baseline T score.
• Perimenopausal women or women who received their first bone mineral density test before age 65. Perimenopause and menopause may trigger rapid bone loss, which may be as much as one T-score point (ie, 1 standard deviation) at the spine and femoral neck.¹⁶ Therefore, testing done during this time cannot be used as the basis of future monitoring.