Timing of follow-up densitometry in hormone replacement therapy users for optimal osteoporosis prevention.

The objectives of the study were (1) to determine the time interval for repeat dual-energy X-ray absorptiometry (DXA) to detect significant bone loss, i.e., greater than the coefficient of variation (CV) of the center (2.8 x CV%) and (2) to assess how long hormone replacement therapy can be maintained to avoid undetected development of low bone mass and to not unduly delay appropriate treatment. A total of 3,826 healthy women, aged 40-65 years, participated in a prospective cohort study, 807 of whom were treated with transdermal estrogen replacement therapy and 626 with transdermal estrogen/progesterone regimens. The untreated group included the remaining 2,393 women. Between 1996 and 2002 they underwent a baseline DXA scan, and DXA scans were then repeated annually. There were no differences among the study groups at entry into the study. Treatment with estrogen was a protective factor for loss of bone mass at the lumbar spine (odds ratio [OR] =0.431, 95% confidence interval [CI] 0.344 to 0.522) and at the femoral neck (OR =0.433, 95% CI 0.352 to 0.521). Treatment with estrogen/progesterone also showed a protective effect against significant changes in follow-up BMD (>2.8 x 1.05% CV of densitometry at L1-L4, >2.8 x 2.3% CV at the femoral neck). In the treated group, significant differences in BMD at the lumbar spine (OR =1.593, 95% CI 1.423 to 2.355) did not appear within the first 3 years, and differences in BMD at the femoral neck (OR =3.555, 95% CI 2.782 to 4.905) did not appear within the first 4 years. It is concluded that in women aged 45-65 years, receiving transdermal hormone replacement therapy without risk factor for loss of bone mass, such as age < 55 years and body mass index <25 kg/m(2), periodical follow-up densitometries would not be necessary, provided that the duration of estrogen or estrogen/progesterone therapy is shorter than 3 years.