To Your Health

A. Duel Energy X-Ray Absorptiometry, or DEXA for short, is considered the gold standard for bone health assessment because it is currently the best available test for predicting fracture risk. DEXA measures bone density, which is related to the amount of mineral in bones. Comparing your bone density to the bone density of young people about age 30, before they start to lose bone minerals, gives a number called a "T-score" that represents your statistical probability of developing osteoporosis. DEXA cannot predict who will experience a fracture but can provide indications of your risk of breaking a bone.

Alternative measurements of bone density include quantitative computed tomography. A standard CT scanner is used, but the amount of radiation exposure is higher than with DEXA and it costs more. An instrument that measures broadband ultrasound attenuation and combines this measurement with measurement of the speed of sound through bone is a relatively new tool to measure bone density. The results are not as precise as with DEXA or QCT, but the instrument has two advantages: It can be used in a physician's office, and it involves no radiation.

There are some limitations to DEXA testing. One is the question of sensitivity. Large increases in fracture risk have been observed with only small decreases in bone density. Bone density tests should be evaluated in conjunction with other independent risk factors for fracture, such as family history of fracture, hormone status, and body weight. Another limitation is that DEXA may not be the most appropriate way to judge the effects of exercise on bone health.

DEXA is often used to monitor bone density after starting an osteoporosis medication such as bisphosphonate. Unfortunately the initial changes in bone density readings are often small and may be a year or two in coming. A decrease in bone density does not necessarily mean treatment failure.

Because changes in bone density are slow, DEXA is generally not done more than once a year at most. Various tests are available that reflect bone loss or gain sooner than a year. Urine tests for a collagen breakdown product called NTx and another for a unique collagen amino acid (hydroxyproline) are useful additions to DEXA. These tests can be repeated as often as every month or so to closely monitor the response to medication.

Bone density is not quite the same as bone strength. Bone strength depends on both bone density and bone structure, so bone density measurement cannot provide a complete assessment of bone strength. Although not yet reliable enough for clinical use, there is an instrument that measures the "bending stiffness" of long bones. Known as "mechanical response tissue analysis," the technique uses painless low-frequency vibration to measure bone strength directly. Until this becomes commercially available, you might be able to get more information from Larry E. Miller, Ph.D., in Flagstaff, Ariz. E-mail millerle@vt.edu.