By the way, doctor: How often... Health Article

By the way, doctor

Q. I'm a healthy 65-year-old woman and just had my first bone density test. How often do I need to have it repeated?

A. Most doctors and organizations recommend that healthy women undergo bone mineral density (BMD) testing at age 65. There's good scientific evidence that BMD is linked to risk of fracture. The most widely used bone mineral density test is dual-energy x-ray absorptiometry, or DXA (see photograph).

How do I interpret my BMD? The World Health Organization defines osteoporosis and osteopenia (bone density somewhat lower than normal) by calculating a woman's T-score, the number of standard deviations below the average value observed in healthy young adults. A T-score of –1.0 to –2.5 has been designated osteopenia. A T-score of –2.5 or lower indicates osteoporosis. If you have osteoporosis, you should seriously consider countering bone breakdown with antiresorptive treatments, such as bisphosphonates, raloxifene, or nasal calcitonin. Estrogen (taken with a progestin in women who have an intact uterus) increases bone mineral density but is generally not prescribed for osteoporosis because of its serious risks — notably, heart disease, stroke, and breast cancer. Women with osteopenia may also want to consider treatment, depending on their other risk factors for fracture. All postmenopausal women should take adequate calcium (1,200–1,500 mg/day) and vitamin D (current dietary guidelines suggest 400–600 IU/day of vitamin D, but recent evidence favors a higher daily intake, and many experts recommend 800–1,000 IU/day). It's also important to get regular weight-bearing and muscle-strengthening exercise.

The purpose of BMD testing is to diagnose osteoporosis or moderate to severe osteopenia in asymptomatic women or those who have had a nontraumatic fracture. It is also used to monitor women who are being treated for osteoporosis. Until recently, there has been little evidence on whether repeat testing is useful for predicting nontraumatic fractures in women your age, but findings reported in January 2007 may change that.

In the prospective Study of Osteoporosis Fractures, researchers at Kaiser Permanente Center for Health Research measured total hip BMD in 4,124 healthy women ages 65 and over. They repeated the test an average of eight years later, then monitored the women for five more years.

During the follow-up, 877 women had non-spine fractures, and 340 had spine fractures. But the investigators concluded that repeat BMD testing didn't help predict the risk of either spine or non-spine fractures. In other words, the first test was just as accurate as the second one, done eight years later. Results were published in the Jan. 22, 2007, issue of the Archives of Internal Medicine.

The study authors noted a few exceptions to their conclusion. If a woman in this age group (65 and over) has lost significant weight, sustained a serious illness, or taken certain drugs — such as steroids, which can accelerate the rate of bone loss — since the first BMD screening, a follow-up measurement might predict her risk for fracture more accurately.

Moreover, the study's conclusions don't necessarily apply to younger women in the early stages of menopause, when the rate of BMD change is less predictable. Most clinicians agree women should be screened before age 65 if they have risk factors for low bone density, including low weight and body mass index, estrogen deficiency, a family history of osteoporotic fracture, Caucasian race, insufficient calcium and vitamin D intake, smoking, and alcohol overuse. How these risk factors should modify screening recommendations is still under debate, but in any case, you should consider BMD testing in the following circumstances:

• You are age 65 or older.

• You are age 50–64 and weigh less than 132 pounds.

• You are age 55–64, weigh 132–154 pounds, and are not taking estrogen.

There's no scientific evidence to guide follow-up screening in younger women. But most clinicians agree that it's rarely necessary more than once every two years, even for women who are being treated for osteoporosis with medications such as bisphosphonates.

— Celeste Robb-Nicholson, M.D. Editor in Chief, Harvard Women's Health Watch