Bone Density Scan Not a Screening Tool

Michele Kohli, BSc, MSc

The Osteoporosis Society of Canada estimates that 1.4 million Canadians have osteoporosis (OP). As discussed in the Clinical Practice Guidelines for the diagnosis and management of osteoporosis,¹ the Society recommends several treatments to improve bone mineral density (BMD) and decrease an individual's risk of fracture.¹ Since BMD loss occurs in all people as they age, the challenge is to decide which individuals have a low enough BMD to warrant preventive treatment. The Osteoporosis Society of Canada endorses using the World Health Organization definition of OP to decide whether or not BMD loss is significant enough to increase the risk of fracture. This definition utilizes the spectrum, or distribution, of BMDs found in young adults. Any individual whose BMD is at least 2.5 standard deviations below the mean for this distribution is said to have OP.¹

Several risk factors for OP and OP-related fractures have been identified, including: older age, female gender, low body weight, cigarette smoking, family history of fracture, history of fragility fractures, loss in height, hyperthyroidism, immobility/ inactivity, calcium or vitamin D deficiency, use of certain pharmaceutical agents (benzodiazepines, anticonvulsants, corticosteroids, heparin) and alcoholism. These risk factors only account for about one third of the risk of having an OP-related fracture. Therefore, BMD measurement is still considered the gold standard test for diagnosis.^1,2,3 However, risk factor analysis may help physicians decide which patients should have BMD measured. The Osteoporosis Society of Canada has also identified men with hypogonadism as being at high risk for OP (see related article Prevention Key, Yet Most Men Can't ID the Risks).¹ Kanis et al. (1997) recommend taking into account risk of falling, nutrition, and life expectancy, when deciding whether or not to assess and treat patients over the age of 75 years.⁴

There are several methods of BMD measurement currently available, all of which have similar accuracy error rates of 3 to 15 percent and precision error rates of 1 to 4 percent. Accuracy refers to the ability to detect the true BMD of the person being scanned, while precision refers to the ability to repeatedly detect the same BMD for an individual in multiple tests. A good diagnostic test should be both accurate and precise.

Absorptiometry techniques use the differential absorption of either photons or x-ray beams in body tissue to estimate bone density. Single photon absorptiometry (SPA) and single x-ray absorptiometry (SXA) can only determine cancellous BMD in peripheral sites such as the radius. The procedure is generally acceptable to patients because scanning time averages only 5 to 15 minutes. Results from these two types of scans are generally reported in grams of bone mineral per centimetre of bone.³ Dual photon absorptiometry (DPA) allows scanning of hip and spine bones, but scanning times average 20 to 40 minutes.³ Dual X-ray absorptiometry (DEXA) is replacing DPA because it allows scans of the hip, spine and peripheral bone sites, with scanning times of only 5 to 10 minutes.³ For both DPA and DEXA, BMD is expressed as grams per square centimetre.³ Most centres that regularly treat patients with OP use DEXA equipment to measure BMD.

The results of all of these scans should include a statement indicating how many standard deviations the person's BMD is from the mean of young adults. Standard x-ray techniques are not reliable enough for use in determining BMD, but may be useful to clarify results from patients with diseases such as degenerative arthritis, which can cause nonstructural calcification.¹

Dr. Tim Murray, Director of the Metabolic Bone Clinic at St. Michael's Hospital, Director