by Marcelle Pick, OB/GYN NP
Dual-energy x-ray absorptiometry (DEXA) technology was introduced in 1988 and has become the most popular tool for measuring bone density.
The National Osteoporosis Foundation recommends that all women over age 65, and post-menopausal women with at least one risk factor for osteoporosis, undergo a bone density test, which is usually a DEXA. Results from bone density tests are used to diagnose osteopenia and osteoporosis.
DEXA measures the bone mineral content (BMC) of the spine, hip, wrist, femur, or any other selected part of the skeleton. It does this by focusing an x-ray on a body site and measuring the proportion of light rays that pass through the tissue as opposed to being blocked by minerals in the bone. Using computer software, it then divides that number by the surface area of the bone being measured to create bone mineral density (BMD).
Bone density test results
After the bone density scan, a picture is printed out that shows where the patient’s BMD falls compared to the norm. The “norm” refers to the accepted standard peak bone mass (PMB) set by a selected reference group of young adults. There are no international standards of PMB because bone density varies so dramatically according to race and region.
Women are given a diagnosis of osteopenia if their T-score (see below) bone density deviates 1.0-2.4 points (standard deviations) below the norm (although we’ve seen patients who were put on Fosamax at just 1.3). Anywhere below 2.5 standard deviations is diagnosed as osteoporosis.
When you are measured against the younger reference group, it is called a T-score. When you are measured against the average BMD for your age, sex, weight and ethnic or racial origin, it is called a Z-score. Everyone loses bone density as they age, so someone with a normal Z-score might deviate significantly from the T-score. However, T-scores are the gold standard for traditional diagnosis. This means that eventually all women’s T-scores will deviate from the norm, and women with natural bone loss might appear to be suffering from a diagnosable disease. No wonder there’s an osteoporosis epidemic!
Limitations of bone density tests
Errors in DEXA measurement can be 8–10% depending on the machine — that’s almost one standard deviation! This means that you might have normal bone density by one machine, and osteopenia by another. It also means that measurements of change in bone density over time are only completely accurate if all your tests are done with the same machine.
Another problem with bone density tests is that the range of healthy bone is much greater than this test would lead women to believe. What’s “low” for one woman may be just fine for another, depending on the thickness of her bones, her ancestry, her peak bone mass from when she was in her 20’s, and other variables.