Medicare pays for bone mineral density testing every two years in older women, but that interval could be longer-even as long as 15 years-in older women with healthy bones, according to a new study. The study, which found that bone loss occurs very slowly in older women who have normal baseline bone mineral density scores and have had no serious fractures, provides new data to help set practical guidelines on rescreening older women.

The researchers followed 4,957 women ages 67 years and older for about 15 years. The women didn't have osteoporosis, as confirmed on dual-energy X-ray absorptiometry (DXA) scanning, or a history of hip or spine fracture when they started the study. Fifteen years later, only about 1% of women with normal bone density scores had developed osteoporosis. Among women with mild or moderate osteopenia, 5% and 21%, respectively, developed osteoporosis within 15 years. But 62% of women with the lowest bone density scores progressed to osteoporosis during the study.

"If a woman's bone density at age 67 is very good, she doesn't need to be rescreened in two years because we're not likely to see much change," lead author Margaret Gourlay, assistant professor at the University of North Carolina, Chapel Hill, Department of Family Medicine, told AJN. She also stressed that the results "do not apply to women under 67." No comparable study of younger women has been done, but Gourlay hopes to study them in the future.

The data were collected as part of the Study of Osteoporotic Fractures. Throughout the 15-year study period, all women had their bone mineral density tested at least twice (as many as five times in some).

Women were assigned to four groups according to baseline T score (a rating of the bone mineral density as it compares with that of a matched healthy adult) at the hip: normal, mild osteopenia, moderate osteopenia, and advanced osteopenia. The researchers calculated how long it took for 10% of women in each group to develop osteoporosis. This took an average 17 years in the normal bone density and mild-osteopenia groups, compared with 4.7 years in the moderate-osteopenia group and 1.1 years in the advanced group. The results suggest that bone mineral density testing could be deferred for up to 15 years among some women with T scores higher than -1.5 because the chance of developing osteoporosis within that time is low.

"The most important message of the study," said Jane Cauley, a nurse and professor of epidemiology at the University of Pittsburgh and a coauthor of the study, "is determining when an elderly woman needs a second bone mineral density scan." Primary care providers and obstetrician-gynecologists order most DXA scans, as do advanced practice nurses working in these specialties. They can apply the new results to avoid ordering unnecessary scans. However, should an older woman with normal bone density develop a chronic condition that requires high doses of corticosteroids, which rapidly degrade bone density, "then you'd want to repeat bone scans sooner," cautioned Cauley. She also pointed out that in the study the interval to osteoporosis did shrink with increasing age.-Carol Potera

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