Bone density testing

Patient information: Bone density testing

Author
Michael Kleerekoper, MD
Section Editor
Clifford J Rosen, MD
Deputy Editors
Constanza Villalba, PhD
Jean E Mulder, MD

Disclosures

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, defined as reduced bone mass and poor bone quality. The disorder makes bones weak and prone to fracture. People who have osteoporosis have bones that can break with even the mildest impact. For example, people with osteoporosis can break a bone just from a minor fall, such as tripping on a loose rug in the living room.

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 need 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 how strong the bones are. 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
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 x-ray detector will detect any metal on your clothing (zippers, belt buckles), so you may be asked to wear a gown for the test.

If you are unable to lie on an examination table, it is impossible to measure the spine and hip bone density. Instead, you can sit beside the DXA machine for a scan of your wrist area. When the hip and spine cannot be measured, the diagnosis of osteoporosis can be made using a DXA measurement of the wrist.

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 1).

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”.)

Low bone mass (osteopenia) — Low bone mass (osteopenia) is the term healthcare providers use to describe bone density that is lower than normal but 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 low bone mass are usually advised to take steps to prevent osteoporosis. Sometimes that includes taking medications. (See “Patient information: Osteoporosis prevention and treatment”.)

Osteoporosis — People with osteoporosis have a T-score of -2.5 or less. Larger numbers (eg, -3.2) 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.

 

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

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

The following organizations also provide reliable health information.

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/aboutosteoporosis/detectingosteoporosis/bmdtest)

The World Health Organization Fracture Risk Assessment Tool

(www.shef.ac.uk/FRAX/)
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)

REFERENCES
Binkley N, Bilezikian JP, Kendler DL, et al. Summary of the International Society For Clinical Densitometry 2005 Position Development Conference. J Bone Miner Res 2007; 22:643.
Hodgson SF, Watts NB, Bilezikian JP, et al. American Association of Clinical Endocrinologists medical guidelines for clinical practice for the prevention and treatment of postmenopausal osteoporosis: 2001 edition, with selected updates for 2003. Endocr Pract 2003; 9:544.
Raisz LG. Clinical practice. Screening for osteoporosis. N Engl J Med 2005; 353:164.
The National Osteoporosis Foundation Clinician’s Guide to Prevention and Treatment of Osteoporosis. file://www.nof.org/professionals/NOF_Clinicians_Guide.htm (Accessed April 28, 2008).

Help improve UpToDate. Did UpToDate answer your qu