Detection

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.

Who should be screened? Currently, screenings for osteoporosis are not routinely given to everyone; instead, they are done on an individual basis. Experts are still debating who should receive bone density screening, and it remains unclear whether the benefits of tests such as dual energy x-ray absorptiometry (DXA) justify the cost of testing everyone. It's best to talk to your doctor about whether testing is right for you. The National Osteoporosis Foundation recommends bone density tests for: all women ages 65 and older all postmenopausal women under age 65 who have one or more risk factors for osteoporosis — other than simply being white, female, and past menopause (see "Who gets osteoporosis?" for a list of risk factors) postmenopausal women who come to their doctors with a broken bone women who are considering osteoporosis therapy, if the test would help them make a treatment decision. In addition, men and women who have taken glucocorticoids for at least two months and individuals who have a medical condition that places them at high risk for osteoporotic fractures should consider getting screened. Adult men who have a history of broken bones should also discuss screening with their doctors. However, it's important to note that coverage varies among insurance plans; some plans may refuse to pay for DXA testing. Thus, unless you're willing to foot the bill yourself, it pays to check with your plan first. For those who are covered by Medicare, the Medicare Bone Mass Measurement Coverage Standardization Act, which went into effect in 1998, allows for bone mass measurement for postmenopausal women, men or women with vertebral abnormalities, patients on long-term steroid therapy, and people with primary hyperparathyroidism. It also covers bone mass measurement every two years to monitor a patient's response to osteoporosis medications.

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.

How likely are you to break a bone? Because the hazards of osteoporosis aren't truly felt until an individual breaks a bone, the ultimate focus of diagnosis and treatment is on predicting who is at risk for fractures and taking steps to prevent them, such as reducing the likelihood of falling (see "Preventing falls"). While bone mineral density tests can identify people who are at greater risk for fractures, they aren't the only predictors. Other factors can increase your chance of falling and breaking a bone. If any of the following red flags apply to you, discuss them with your doctor: low levels of physical activity low muscle mass or impaired strength balance problems poor eyesight excessive alcohol use a history of falls the presence of environmental hazards, such as electrical cords or throw rugs in walking paths the use of medications, such as sedatives and blood pressure drugs, that can cause dizziness, lightheadedness, or impaired balance advanced age.

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 Hunting for osteoporosis Ultrasound screening Ultrasound bone density scan 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.

Biochemical markers

Several companies have developed blood and urine tests that indicate the rate at which bone is being formed and destroyed by measuring the markers of bone turnover. While these tests are sometimes used, the information they can provide is limited, and they can't tell you if you are at risk of breaking a bone.

Some of these tests measure collagen cross-links — proteins contained in the structural framework of bone, which are released during bone resorption. Others measure alkaline phosphatase and osteocalcin, proteins that are instrumental to bone formation. These tests can indicate high bone turnover, which can be a sign of rapid bone loss. However, they cannot indicate bone mass any more than measurements of metabolic rate can indicate a person's weight. Like metabolism, bone turnover varies from person to person and from time to time. Thus, a turnover rate that may point to bone loss in one person may represent the status quo for another.

These tests are sometimes used to provide an early indication of whether someone is responding well to treatment for bone loss. A urine test revealing that bone turnover has slowed following treatment can be seen as a sign that therapy is having an effect. But it's not clear if this translates into slower bone loss or improvement in bone density. Conversely, test results showing that the rate of bone turnover has remained the same or increased may indicate that the treatment is ineffective or that the patient is not taking the medication.

Don't give up on therapy While bone mineral density (BMD) tests are often used to monitor how a patient is responding to treatment, one study concluded that an initial loss of BMD after beginning drug therapy may not be cause for concern. The study, published in 2000 in the Journal of the American Medical Association, indicated that some patients lose BMD after their first year of therapy, only to gain it during their second year. The study's message was that patients should stick with treatment even if an initial BMD test shows less than stellar results. Because losses are sometimes the result of patients not taking their medications, a poor showing should prompt doctors to talk with their patients. But the losses may also reflect individual variations in bone density, imperfections in testing methods, or random errors.

But markers can't reveal fracture risk, and they seem to correlate only dimly with changes in bone mass in an individual. Currently, they can't routinely provide you with enough information to make a decision about whether to undertake or forgo preventive treatment, but they may be helpful under certain circumstances when bone turnover needs to be assessed.

Source: from Harvard Health Publications, Copyright © 2008 Harvard University. All rights reserved. Harvard Medical School does not endorse products.
Used with permission of StayWell.
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