Bone health
Bone mineral density testing and bone scan results
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!
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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 no two women are the same, even though the test holds them to the same norm. A larger-boned woman may have more bone tissue, thus more minerals, and may score a higher bone density than a smaller-boned woman. Other variables like your ancestors’ bone density and your own peak bone mass from when you were in your 20’s affect your ideal bone density. The range of healthy bone density is so much greater than this test would have women believe — which can make the test results misleading.
Furthermore, reference groups that set the “norm” change from institution to institution. The most insightful DEXA results come from comparison to a reference group of people of the same race, age, region, and gender as yourself.
Several studies in Canada, Turkey, and the third National Health and Nutrition Examination Survey here in the US compared BMD samples from a diverse but local population with those supplied by DEXA’s manufacturers. The results showed that the percentage of women who would be called osteoporitic by the manufacturer’s standards dropped 20% under the customized standards.
The worrisome thing about these discrepancies in bone density test results is that women are changing their behavior dramatically when they think they’re abnormal. When a test indicates low bone density, they feel frightened and immensely fragile. Often women stop lifting and exercising for fear of a fracture — when that’s just what they need to be doing to support bone health! They also may start a course of preventative — and speculative — osteoporosis drugs that may continue for the rest of their lives.
So look at your bone density test with a healthy dose of skepticism. For a bone density test to truly be useful, you would need to know your own baseline bone density at age 25–30 and compare it 30 years later to a reading from the same machine.
Even then, there is more to bone health than simple bone mineral density. Bone density testing is one of the best tools we have as an osteoporosis test, but keep in mind that it provides only a sliver of information about overall bone health.
Click here to return to our article on osteoporosis and Fosamax.
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References & further reading on bone health
Original Publication Date: 09/17/2005
Last Modified: 09/15/2008
Principal Author: Marcelle Pick, OB/GYN NP