Should you have a bone density test?

Physicians don’t recommend that every woman over 50 be tested for osteoporosis, though those who are at risk should consider it. A person’s risk factors "should help her, in consultation with her physician," says Dr. David A. Hanley, head of the Division of Endocrinology and Metabolism at Foothills Hospital in Calgary, "to decide that maybe she should be screened for osteoporosis, and we have very good tests for assessing someone’s bone density."

Those techniques don’t generally include blood tests, though there is a specific blood test under investigation at the moment. Unfortunately, it’s expensive, and the results can vary widely in an individual within a given day, and from day to day, so a single test doesn’t really tell the doctor a great deal. Where blood tests are useful is after bone loss has been established, as a means of ruling out anything else that could be causing the loss, such as cancer or thyroid disease.

Normal x rays aren’t used in bone mineral densitometry, or BMD, as bone density testing is called, because they can’t pick up osteoporosis until at least 30 per cent of the bone mass is already gone, at which point the disease is fairladvanced. There are other scanning techniques that are better, including dual photon absorptiometry (DPA), quantitative computer tomography (QCT) and – the gold standard – dual energy x ray absorptiometry, or DEXA.

With DEXA, the patient lies on a table while a scanner passes over her, measuring the density of the lower (lumbar) spine and hip. The lowest density reading – measured as a standard deviation (SD) below the mean density for young adults – is used to determine whether therapy is required. A reading within 1 SD is considered normal, 1 to 2.5 SDs below the mean indicates osteopenia (reduced bone mass, a precursor to osteoporosis), and more than 2.5 SDs below the mean indicates osteoporosis.

DEXA’s relatively inexpensive ($75 to $200), painless and quick; it uses very low dose x rays, and produces the most accurate assessment of any of the current techniques; the drawback is that DEXA machines aren’t widely available.

A system involving ultrasound to measure density in the large bone of the heel (the calcaneus) is also available. It’s portable, cheap and doesn’t involve any radiation, and it focuses on an easily accessible area of the skeleton that contains a disproportionate amount of trabecular bone, the bone most involved in osteoporosis. It looks promising, though it has yet to be validated by long term studies against DEXA.

Hawker sees the two techniques as complementary. "Right now," she says, "I think the recommendation would be, if you get screened with an ultrasound [and you have a positive result], you have a confirmatory DEXA. It may be that we find out in the next few years that the combination of DEXA and ultrasound is in fact the most predictive, but at this point in time I would never, ever treat anybody on the basis of ultrasound alone."

Again, should you be tested? Yes, if your risk factors put you in a high risk category. You may also consider having a BMD test done if you’re having trouble deciding whether or not to take hormone replacement therapy.

"If you’ve already decided to take estrogen, you don’t need to have a bone density test," says Dr. Robert Josse, chief of the Division of Endocrinology and Metabolism at St. Michael’s Hospital in Toronto, "because estrogen is the treatment for osteoporosis, and it’s certainly the preventive treatment used by all the osteoporosis agencies around the world. But if you’re wavering about estrogen therapy, the Osteoporosis Society of Canada endorses the use of bone densitometry to empower you to make the decision."