What is a Bone Density Test?
Low bone density is one of the important risk factors for osteoporosis. It alone does not predict whether you’ll have a future fracture, but it is one of the four or five high-ranking risk factors. Currently, bone density testing is considered the best way to determine if a person has osteoporosis, osteopenia (density lower than the expected norm, but less than the level used to determine osteoporosis), or normal bone density for their sex and age. Sequential bone density tests done over several years is one means to determine an individual’s overall bone health.
There are several different ways to assess the health of your bones. The most popular method is by looking at your bone density to see if it is adequate for your gender and age. Common methods of measuring bone density include dual-energy x-ray absorptiometry (DEXA); single-photon absorptiometry (SPA); broadband ultrasound attenuation (ultrasound); and quantitative computed tomography (QCT).
Bone density screening tests
The best known bone-density test is dual-energy x-ray absorptiometry, or DEXA Scan, which is one of the most precise tests and is the gold standard for most physicians. The SPA (single-photon absorptiometry) is a radiation scanning technique that measures bones such as those in the wrist or heel. It doesn’t tell us about the density of bones that are most likely to fracture from osteoporosis, such as the hip and spine. Measurements of the heel, however, are good initial screening tests for possible osteoporosis of the spine and hip. Ultrasound uses sound waves to estimate the strength of certain bones, such as those in the heel, wrist, or finger. Finally, the QCT is another radiation-based technology that measures the central bones such as spinal vertebrae.
All of these tests attempt to assess the density of your bones. Results are generally given in numbers as grams per cubic centimeter and also calculated as a “T score” and a “Z score.” Your T-score compares your bone density with that of an ideal young standard of the same gender. The Z-score compares density with that of other people your same age and sex. Here’s how the tests differ:
- The DEXA commonly measures the bone density of the spine and the hip. It is the most precise and widely used of all existing tests.
- SPA is a simple radiation scanning technique used to measure “appendicular” bones, such as the wrist or heel. As with ultrasound, measurements of the wrist, finger, or heel are good screening tests, but they do not necessarily tell us about the density of the bones that are most likely to fracture — the hip and spine.
- Ultrasound is a simple technique using sound waves to estimate strength of certain bones such as those of the heel, wrist, shinbone, or finger. Ultrasound measurements are best seen as “screening tests” that reveal the possibility of low bone density and bone weakness. There is often, but not always, a fairly good correlation between ultrasound measurements of the heel and density of the hip or spine.
- QCT is another radiation-based technology that directly measures the “central” bones such as the spinal vertebral bodies. This test involves more radiation exposure and is more technically involved than the other tests.
The limitations of BMD testing
Steps you can take to limit variability in BMD tests
- Always schedule your BMD test for the same time of year. Bone density fluctuates with the seasons, particularly in the northern hemisphere where sunlight and vitamin D production decline in winter months.
- Always have your BMD test done at the same center and on the same machine. If possible, you should even try to have the same operator each time. This will limit variation because of machine calibration and operator skill.
- Make sure you ask the center to provide you with the complete test results, including pictures, rather than providing you with a summary. A summary uses an average of various sites’ measurements, which can be misleading; it’s better to compare the measurements for specific sites over time to get a sense of whether you’re losing bone consistently in all sites.
While the bone density measurement indirectly estimates the mineral content of bone at any given time, the tests of bone mineral density do not reveal what is currently happening within bone. For example, your DEXA might say you have low bone density as compared with the standard young reference range, but this does not give a sense of whether you have had thin bones since youth, or whether something caused you to lose bone after reaching a peak bone mass. A single bone density measurement also cannot distinguish if you are currently losing bone and thus experiencing ongoing bone density reduction. Moreover, the bone density test results can vary depending on the season, the machine used, and even differences in the way the technician operates the machine and interprets the results. The tests are therefore best used as a way of looking for ongoing changes in bone over time.
Also, it’s important to realize that body type matters: people who are naturally thinner will always have lower bone density than their heavier counterparts. This does not mean they will automatically develop osteoporosis or fracture, but it does mean that in conjunction with other risk factors, such as poor diet, smoking, and a worried or anxious outlook on life, such thin people may be at a disadvantage when it comes to fracture risk. In addition, bone mineral density testing cannot detect either bone quality or bone strength, which are the all-important factors in fracture risk.
At the Center for Better Bones, we look for trends over time in bone density, but don’t get overly concerned with small variations. To be truly significant, there must be at least a 6% change in bone density — anything else could simply be operator error or seasonal changes. Significant changes are rarely seen in less than two years, and some European and Canadian researchers are now suggesting that bone density testing be performed only every five years for most people.