Osteoporosis: Diagnostic Tests

researched and written by the ProjectAWARE group, 2001

Currently there is no accurate measure of overall bone strength. Bone mineral density (BMD)—see chart below—is frequently used as a proxy measure and accounts for approximately 70 percent of bone strength.³³

Several methods are available to measure bone density, but currently the most widely used technique is DEXA (Dual Energy Xray Absorptiometry). This is the method used to determine efficacy in the recent large clinical trials and to characterize fracture risk in large epidemiological studies. Older methods such as single photon absorptiometry do not predict hip fractures as well as DEXA.⁶

There is a need to improve the reporting of bone mineral density (BMD) and fracture risk so it is understandable to medical specialists and can be explained to patients. The value of bone density in predicting fracture risk is established, and there is general consensus that bone density measurement should be considered in patients receiving glucocorticoid therapy for 2 months or more and patients with other conditions that place them at high risk for osteoporotic fracture.³³

Diagnostic categories depend on the bone density and the presence of fractures. The WHO (World Health Organization) committee set cut-off values that were relative to young healthy individuals between 25–30 years of age who are considered to have peak bone density. The T-score definitions involve "standard deviations" which are statistical units of variation. Osteopenia, a level considered only "somewhat low", is defined as a bone density between one standard deviation and 2.5 standard deviations below average for young people. Osteoporosis is a bone density lower than 2.5 standard deviations below young people. Established osteoporosis is a bone density lower than 2.5 standard deviations in the presence of fragility fractures.⁶

• Normal bone: T-score better than -1.
• Osteopenia: T-score between -1 and -2.5
• Osteoporosis: T-score less than -2.5
• Established osteoporosis includes the presence of a non-traumatic fracture.

Bone Density	Normal	Osteopenia	Osteoporosis	Established
Cortical bone
Trabecular bone

Z-score, not shown here, is the number of standard deviations from the value of ambulatory individuals of the same sex and age.⁶

Bone density measurements can be done at the hip (proximal femur), total body, spine, radius, and calcaneus. The total hip has the best ability to predict hip fractures and can predict spine fractures as well as the spine density. With newer techniques the hip is the best site to measure. Spine measurements are frequently inaccurate because of osteoarthritis, aortic calcifications, or other skeletal problems such as surgery or compression fractures. This is especially true in elderly patients. Many centers measure both the spine and the hip, but this is usually unnecessary and will certainly increase the cost.⁶

With most antiresorptive therapies there is a greater change at the spine than at the hip. The spine has a higher percentage of trabecular bone that is more metabolically active. However, both bisphosphonates and estrogens will increase bone density at the hip as well as the spine, and thus this site can also be used to follow most patients.⁶

Some investigators have argued that both spine and hip should be measured to achieve greater sensitivity in prediction of fractures. A large study which prospectively measured fracture incidence documented that this is not true. To increase sensitivity (to detect more patients who might get a hip fracture) one could measure just the hip and choose a higher threshold.⁶

There are some clinical situations in which the spine and the hip would biologically be different, and in these infrequent cases measurements of both the spine and the hip are justified. The most common is hyperparathyroidism, both primary and secondary, in which the bone density at the hip is more affected than at the spine. This is because parathyroid hormone (PTH) tends to increase bone loss from cortical bone more than from trabecular bone. In fact, PTH may increase trabecular bone mass while decreasing cortical bone mass. Fluoride also preferentially increases trabecular bone mass. Corticosteroids, on the other hand, have a greater effect on the trabecular bone, but spine fractures may falsely increase the spinal bone density.⁶

Next: Therapies: Current, New & Experimental

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Updated 05/16/2010