Patient information: Bone density testing

WHAT DOES BONE DENSITY TESTING DO AND WHY IS IT IMPORTANT?

People tend to think that bones are static and unchanging, but the truth is that bones are in constant flux. Even as you read this sentence, specialized cells in your body are busy destroying old bits of bone and replacing them with new bone. Unfortunately, as people age, they often lose bone more quickly than they can replace it, so their bones can become porous and brittle (figure 1).

If left unchecked, this bone loss can lead to a disorder called osteoporosis, which weakens bones so much that they can break with even the mildest impact. For people with osteoporosis, something as innocuous as tripping on a loose rug in the living room can spell disaster.

Each year in the US, osteoporosis leads to 1.5 million fractures, including:

• 700,000 fractures of the vertebrae, the bones in the spine,
• 300,000 hip fractures,
• 250,000 wrist fractures, and
• 250,000 fractures of other parts of the body.

Fractures of the spine and hip can lead to chronic pain, deformity, depression, disability, and even death. Plus, half the people who break a hip never regain the ability to walk without assistance and a quarter wind up needing long term care.

The problem is, osteoporosis does not cause any symptoms, so people do not usually know they have the condition until they break a bone unexpectedly. That's where bone density tests come in.

Bone density tests measure the minerals that are in a person's bones. They reveal, roughly speaking, how strong a bone is. Healthcare providers use these tests to both screen for and diagnose osteoporosis. The tests are important, because they can alert you to problems with your bones before you have a fracture.

If it turns out that you have osteoporosis or are at risk for it (known as low bone mass or osteopenia), you can take steps to prevent fractures. (See "Patient information: Osteoporosis prevention and treatment".)

WHO SHOULD GET BONE DENSITY TESTING?

Osteoporosis targets women much more often than men, and it becomes more common after menopause and with advancing age. As a result, healthcare providers recommend bone density testing for women who have been through menopause and are at least 65 years old. In addition, there are certain characteristics that put people at higher risk for fracture, so healthcare providers sometimes recommend testing in men or women younger than 65 who have one or more risk factors. (See "Screening for osteoporosis".)

Risk factors for fracture — Factors that increase a person's risk of fracture and may lead to earlier bone density testing include:

• Cigarette smoking
• Long-term use of steroid (glucocorticoid) medications such as prednisone
• Low body weight (less than 127 lbs or 58 kg)
• Rheumatoid arthritis
• History of a non-traumatic or low trauma bone fracture in self or parents (eg, breaking a bone after falling from standing height or less)
• Excessive alcohol consumption (three or more servings a day)
• A disorder strongly associated with osteoporosis, such as diabetes, untreated hyperthyroidism, hyperparathyroidism, early menopause, chronic malnutrition or malabsorption, or chronic liver disease

For a summary of the recommendations from expert groups for who needs bone density testing, see figure 2 (table 1).

WHICH TEST IS BEST?

There are several different types of bone density tests.

Dual-energy x-ray absorptiometry (DXA) — Experts agree that the most useful and reliable bone density test is a specialized kind of x-ray called dual-energy x-ray absorptiometry, or DXA. DXA provides precise measurements of bone density at important bone sites (such as the spine, hip, and forearm) with minimal radiation.

We recommend DXA of the hip and spine because measurements at these sites are effective for predicting osteoporotic fracture at any site, choosing candidates for therapy, and for monitoring response to therapy.

Quantitative computerized tomography — This is a type of CT that provides accurate measures of bone density in the spine. Although this test may be an alternative to DXA, it is seldom used because it is expensive and requires a higher radiation dose.

Ultrasonography — Ultrasound can be used to measure the bone density of the heel. This may be useful to determine a person's fracture risk. However, it is used less frequently than DXA because there are no guidelines that use ultrasound measurements to diagnose osteoporosis or predict fracture risk. In areas that do not have access to DXA, ultrasound is an acceptable way to measure bone density.

WHAT TO EXPECT FROM A DXA TEST

During DXA, you lie on an examination table. An x-ray detector scans a bone region, and the amount of x-rays that pass through bone are measured and displayed as an image that is interpreted by a radiologist or metabolic bone expert. The test causes no discomfort, involves no injections or special preparation, and usually takes only five to 10 minutes.

The amount of radiation that's used is minimal, amounting to roughly the same radiation that an average person gets from the environment in one day. After the test is completed and the doctor interprets the results, you will be given a score that speaks to the condition of the bones.

WHAT DO THE RESULTS MEAN?

The results of a bone density test are expressed either as a "T" or a "Z" score. T-scores represent numbers that compare the condition of your bones with those of an average young person with healthy bones. Z scores instead represent numbers that compare the condition of your bones with those of an average person your age. Of these two numbers, the T-score is usually the most important. T-scores are usually in the negative or minus range. The lower the bone density T-score, the greater the risk of fracture (table 2).

Normal bone density — People with normal bone density have a T-score between +1 and -1. People who have a score in this range do not typically need treatment, but it is useful for them to take steps to prevent bone loss, such as having adequate amounts of calcium and vitamin D and doing weight-bearing exercise. (See "Patient information: Osteoporosis prevention and treatment".)

Osteopenia (also known as low bone mass) — Osteopenia is the term healthcare providers use to describe bone density that is lower than normal but that that has not yet reached the low levels seen with osteoporosis.

A person with osteopenia does not yet have osteoporosis, but is at risk of developing it. People with osteopenia have a T-score between -1.1 and -2.4.

In you have other risk factors for fracture (see 'Risk factors for fracture' above, and have a T-score in the osteopenic range, you may be at high risk for fracture. People with osteopenia are usually advised to take steps to prevent osteoporosis, sometimes including medications. (See "Patient information: Osteoporosis prevention and treatment".)

Osteoporosis — People with osteoporosis have a T-score of -2.5 or less. Larger numbers indicate lower bone density because this is a negative number.

The lower the bone density, the greater the risk of fracture. If you discover that you have osteoporosis, there are several things you can do to reduce the chances that you will break a bone. For instance, taking osteoporosis medications combined with calcium and vitamin D supplements and an exercise program can reduce your fracture risk. (See "Patient information: Calcium and vitamin D for bone health" and "Patient information: Osteoporosis prevention and treatment".)

DO I NEED TO HAVE BONE DENSITY TESTING AGAIN?

Even if your bone density test shows that you do not have osteoporosis today, you may need to have the test again. How long to wait between tests depends on your gender and on whether you have risk factors that represent an ongoing threat to your bones.

• If you take medications that decrease bone density or have medical conditions that can adversely affect the bones, experts recommend repeat bone density testing every one to two years.
• For women who have multiple risk factors for fracture (see 'Risk factors for fracture' above, experts recommend repeat bone density testing every two years during the first five years following menopause, when bone loss is most rapid.
• For women who have no risk factors for fracture, experts recommend repeat bone density testing every three to five years.

Other people may also need repeat bone density testing every two years. This includes people who have osteoporosis and begin taking medications to stall further bone loss or to stimulate new bone growth. The results of the follow up tests are used to monitor the effects of the treatment.

SUMMARY

Bone density tests help healthcare providers spot bone loss in people who might otherwise have no symptoms. The tests are painless, quick, and safe, and can alert people to bone loss-before they have a fracture. The tests are also useful in tracking the effects of medications used to manage bone disease.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
Patient information: Osteoporosis prevention and treatment
Patient information: Calcium and vitamin D for bone health

Professional Level Information:
Diagnosis and evaluation of osteoporosis in postmenopausal women
Osteoporotic fracture risk assessment
Prevention of osteoporosis
Screening for osteoporosis

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/ency/article/007197.htm, available in Spanish)

• The United States Surgeon General

      (www.surgeongeneral.gov/library/bonehealth/index.html)

• Osteoporosis and Related Bone Diseases National Resource Center

      (www.niams.nih.gov/Health_Info/Bone/Bone_Health/bone_mass_measure.asp)

• National Osteoporosis Foundation

      (www.nof.org/osteoporosis/bmdtest.htm)

• Osteoporosis Society of Canada

      (www.osteoporosis.ca/)

• The Hormone Foundation

      (www.hormone.org/public/osteoporosis.cfm, available in English, Spanish, French, Italian, German, and Portuguese)

UpToDate wishes to acknowledge Constanza Villalba, PhD, medical writer, for her contributions to this topic.

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