However, even if BMD were an indicator of bone strength, changes are not usually reversible, and very low density might be detected too late for most effective interventions (BMJ, Jul 22 1995).

An earlier study concluded that BMD measurements are a poor screening test for future hip fracture. It found that among all women screened, thr group at highest risk had less than half the fractures over a nine-year peril (Ann Intern Med, 1986; 104:817-23).

A large review of published work on BMD screening has concluded that BMD screening has never been shown to be effective in preventing osteoporotic fracture. Measurements vary at different times and with different equipment, and BMD varies very little between the groups who will and won't go on to suffer fracture (Bristol: Health Care Evaluation Unit, University of Bristol, 1992.)

The majority opinion from North America and the UK is against random screening. Nevertheless, measuring bone mass may be an important tool in the diagnosis of osteoporosis in high-risk groups and for monitoring some patients (BMJ, June 10, 199S). If you get a low reading, it might be a good idea to get a second opinion and also to check the age of the equipment and the experience of the operator.

Safety-wise, DEXA, the most widely used system, uses lower doses of radiation than single-photon absorptiometry (SPA), dual-photon absorptiometry (DPA) and quantitative computed tomography (QCT). The average skin exposure for DEXA is I to 3 mrad per scan (compared to 8-10 mrads for chest x-rays, for example). QCT uses the highest radiation dose.