Detecting Osteoporosis: Osteoporosis

Content provided by the Faculty of the Harvard Medical School

Detecting osteoporosis

Osteoporosis used to be diagnosed only after a bone fracture. For many people, that diagnosis came too late to be of much use. Today, osteoporosis can be detected earlier with a bone mineral density (BMD) test. This test can also provide information regarding your risk of suffering a fracture and can help you and your doctor monitor your progress if you're taking bone-building medications.

BMD is the measurement of a bone's mineral content. In general, the lower your bone density, the higher your risk for fracture. Usually, BMD is measured by calculating the amount of mineralized tissue in grams per square centimeter in the area scanned (for example, the hip, spine, or heel). That number, like the figure that represents cholesterol level, represents a place on a spectrum created from the BMD measurements of people of all ages.

To decide what levels place people at risk, statisticians looked at the bone densities of thousands of women and noted who had osteoporotic fractures and who didn't. Since women who are in their 30s — when bone mass is at its peak — have the lowest fracture risk, their average bone mass was used as the reference point. Researchers then determined the levels of bone density that were associated with increased risk for osteoporosis.

Osteoporosis is defined in terms of standard deviations from the average peak bone mass, also called a T-score. Standard deviation is a statistical term used to express the amount of variation away from the mean, or average. For example, if the average weight of nine people is 125 pounds and one standard deviation is 12.5 pounds, then people who weigh between 112.5 and 137.5 pounds are within one standard deviation of the average. Those who weigh between 100 and 150 pounds are within two standard deviations. The 100-pound woman is said to have a T-score of -2 and a 150-pound woman has a T-score of +2.

The World Health Organization has established the following classification system based on bone density:

• If your T-score is greater than -1: Your bone density is considered normal.

• If your T-score is -1 to -2.5: You have low bone mass (known as osteopenia), but not osteoporosis.

• If your T-score is -2.5 or less: You have osteoporosis, even if you haven't yet broken a bone.

Several technologies can assess bone mass, but two (see Figure 8) have emerged as the most common.

Figure 8: Hunting for osteoporosis DXA screening Ultrasound screening Dual energy x-ray absorptiometry (DXA) and ultrasound are two of the most common methods of detecting osteoporosis. The top photograph shows a patient undergoing DXA screening; the bottom one shows a patient being tested with an ultrasound device. While most physicians consider DXA the most accurate diagnostic procedure, ultrasound devices are becoming more popular thanks to their compact size and low operating costs.

Dual energy x-ray absorptiometry (DXA). For this procedure, a machine sends x-rays through bones in order to calculate bone density. The process is quick, taking only 10 minutes. And it's simple: You lie on a table while an imager passes over your body. DXA is the most commonly used method of assessing BMD. It has emerged as the gold standard of BMD testing. This technology can measure BMD at any spot in the body, but is usually used to measure it at the spine, hip, or wrist, or for the total body. DXA can compute the density of bone in any region of the body, and it does so with only one-tenth of the radiation exposure of a standard chest x-ray.

Ultrasound. Ultrasound, which uses sound waves to measure BMD at the heel, shin, or finger, is increasingly being used as well. It does not give measurements as exact as those provided by DXA, but it seems to predict fracture risk. The process is quite simple. For example, to measure BMD at the heel, you will be asked to place your bare foot in a device (known as a sonometer) that emits high-frequency sound waves. A computer determines the bone density by calculating how fast the sound waves pass through your heel. The machine can provide an estimate of your bone density in less than a minute. Lightweight and easy to use, this small device may make BMD measurements more accessible.

Several other methods can also measure bone density, but they are used less commonly than DXA testing and ultrasound. While the various technologies offer patients and physicians more choices, they also present a challenge for researchers. There are no universal standard measurements or procedures for determining bone density. Researchers have used a variety of different instruments to measure bone density, but the measurements vary according to the equipment. In addition, they have measured bone at a number of different sites — heel, wrist, forearm, hip, and spine — but because bone density varies throughout the body, measurements taken at one spot are not directly equivalent to those taken at another.

Thus, for now at least, bone density is expressed in standard deviations that are specific to the site measured and the equipment used. For this reason, it's best for people who have been using a particular technology to track their bone density to continue using that technique, if possible. Within certain limits, bone density measurements at one site may be used to predict risk at another site.

Last updated:	January 23, 2007