Bone Density Test
Bone Density Test
Bone density test
A bone density test, also called a bone mineral density (BMD) test, is a measurement of bone mass (thickness and sturdiness), usually taken at the lumbar (lower) spine and hip, the areas most likely to fracture due to low bone mass. Tests range from x-ray scans of the wrist or heel performed with small, portable machines—often in stores or pharmacies—to more accurate and sophisticated tests performed with larger machines in hospitals, radiology centers, or some doctor's offices.
The purpose of a bone density test is to evaluate an individual's risk for developing osteoporosis—and, consequently, to assess the likelihood of fracture resulting from the loss of BMD. The National Osteoporosis Foundation lists the major risk factors for developing osteoporosis. These include:
- Age. People over 50 are at increased risk because bones become weaker and less dense with age.
- Gender. Although men get osteoporosis, women are more likely to develop the disease. Women have less bone tissue than men and lose bone more rapidly due to estrogen loss during and after menopause.
- Family and personal history of fractures. Heredity and sustaining a prior fracture as an adult may increase a person's fracture risk.
- Race. Although Caucasian and Asian women are more likely to develop osteoporosis, all postmenopausal women are at risk for developing the disease.
- Bone structure and body weight. Small-boned and thin women (less than 127 pounds) are at increased risk.
- Menopause and menstrual history. Menopause, whether natural or surgical, increases the risk of developing osteoporosis. Also, women who stop menstruating due to disorders like anorexia or bulimia, or because of excessive physical exercise, may also lose bone tissue and develop osteoporosis.
- Lifestyle. Smoking, drinking excessively, consuming an inadequate amount of calcium, or getting little or no weight-bearing exercise, increases the risk of osteoporosis.
- Medications and chronic diseases. Some medications used to treat disorders such as rheumatoid arthritis, endocrine disorders (such as thyroid disease or type 1 diabetes), seizure disorders, and gastrointestinal diseases may experience side effects that lead to bone damage and osteoporosis.
Thus, bone density tests are most often administered to:
- Women 65 and older without risk factors and younger postmenopausal women with risk factors.
- Those with certain medical conditions or a family history of osteoporosis.
- Those taking certain types of medications that affect bone density.
- Those who have had a fracture or whose prior x rays have revealed a bone-density problem.
If low bone mass is detected on a bone density test, the test is usually repeated two to three years later to monitor a person's response to treatment and thereafter at the physician's discretion.
Because the most widely used bone-density tests emit low-dose radiation, pregnant women should discuss any health risks with their healthcare provider before testing.
Bone in humans and other mammals is generally classified into two types. Cortical bone, also known as compact bone, forms the dense, outer surface around the bone at the end of joints and the vertebrae. Trabecular bone, also known as cancellous or spongy bone, is found deep inside the outer shell.
Although there are several types of imaging studies that evaluate bone density, the dual-energy x-ray absorptiometry (DEXA) is considered the “gold standard”—that is, the most accurate—in measuring bone mineral density. Machines called densitometers use two x rays having different energy levels to measure the density of calcium and other minerals in the bone. The denser the bones, the less of the x rays are able to pass through them. These machines are able to detect cortical bone loss of as little as 2% per year and can produce better images than other types of equipment.
The test selected depends on the equipment available, the area of the body to be tested, and the cost. Tests performed on portable equipment are less costly but are also less accurate and are usually used for preliminary screening.
The major types of BMD tests are listed below:
- In the DEXA, which uses less radiation than a chest x ray, a person lies on his or her back on a padded table, then on their side for the 10- to 20-minute procedure. The machine takes pictures of the lumbar spine and hip.
- Peripheral DEXA (P-DEXA) tests, often performed in doctors' offices, measure bone density in the legs and arms only. These are usually performed sitting down.
- Single-photon absorptiometry (SPA) uses a radioactive substance to measure BMD in the wrist or heel and takes about 10 to 20 minutes.
- Dual-photon absorptiometry (DPA), which measures BMD in the spine, hip, and total body, takes about 20 to 40 minutes, and the cost is higher than SPA.
- Quantitative computed tomography (QCT) is a type of CT scan. This test emits more radiation than the DEXA and measures trabecular bone—rather than cortical bone like the DEXA—and only measures BMD in the spine. Although the test, which requires lying down on a padded table, takes about 10 to 15 minutes, it is the most costly BMD test and is not as accurate as the DEXA tests or DPA.
- Peripheral QCT measures BMD in the wrist only.
- Ultrasound testing that uses sound waves, measures BMD in the heel.
There are a few simple things to keep in mind when preparing for a bone density test.
- Wear comfortable clothing that contains no metal buttons, buckles, or zippers, and remove metal jewelry so that changing into a gown before the test will be unnecessary.
- Bring a list of all current prescription, over-the-counter drugs, and vitamins and supplements and include information on dosages, frequency, and duration of use.
- Do not take any medication or supplements containing calcium—including antacids such as Tums or Rolaids, calcium supplements, multivitamins, or medication for osteoporosis for 24 hours before the test.
- Do not schedule a bone-density test within two weeks of a nuclear medicine procedure (like a bone scan) or within one week of any test that uses a contrast medium (like a barium enema or an angiogram).
QUESTIONS TO ASK YOUR DOCTOR
- Why do I need this test?
- How long will the test take?
- Will any medications that I am taking interfere with the test?
- Can I take calcium supplements or vitamins before the test?
- Can I eat and drink normally before the test?
- How much radiation will I be exposed to?
- How do I get the results of my bone density test?
- How often will I need to repeat this test?
There is no aftercare required after a bone density test.
There are usually no complications that result from bone density tests.
The results of a DEXA exam are reported in two numbers: an age-matched number (called the Z score) and young-normal number (called the T score). The Z score compares an individual's results to those of his or her age group. Z scores are used in younger men and in women who are premenopausal with other risk factors.
The T score, which is used in postmenopausal women and in elderly men, compares the results to the “norm,” that is, a healthy 30-year-old of the same sex and race—the age at which peak bone density is achieved. The DEXA results indicate if the bones tested are more (+), less (−), or the same density as a healthy 30-year old. Ranges for interpreting T scores are listed below. The results of BMD tests are noted as “standard deviation” (distance or spread) from the norm:
- Optimal = BMD above +1.0 standard deviation (SD)
- Borderline = between a +1.0 SD and a −1.0 SD
- Low, or osteopenia = between a −1.0SD and a −2.5SD
- Osteoporosis = below a −2.5 SD
Bone scan —An imaging study (such as a magnetic resonance imaging [(MRI] or computed tomography [CT]) used to determine bone abnormalities rather than bone mineral density.
Contrast medium (or agent) —A substance usually injected, swallowed, or used as an enema, before an x-ray image is taken to increase the visual contrast between an internal area of the body under study from the surrounding tissue.
Menopause —The cessation of menstruation.
Nuclear medicine —A subspecialty of radiology that uses small amounts of radioactive material attached to drugs for the purpose of diagnosing or treating certain medical conditions.
Osteopenia —Low bone mass or density.
Osteoporosis —Bone density measured as −2.5standard deviations below the norm of a healthy 30 year old.
If left untreated, osteoporosis can lead to loss of height, fractured or broken bones, a stooped or hunchback position, and severe pain . The National Osteoporosis Foundation reports that osteoporosis affects 10 million Americans, and most of those affected are women.
A physician orders the procedure (often a person's family doctor, orthopedist—if a bone has been fractured or broken—or endocrinologist). The test is often done on an outpatient basis in a radiology center, hospital, or sometimes in a doctor's office. The test results are read and interpreted by a radiologist, then forwarded to the physician who ordered the test.
If test results show osteoporosis, the doctor may prescribe medications and will discuss calcium and other vitamin and mineral supplementation as well as lifestyle issues such as level of activity, other medications, smoking and alcohol intake, and weight. If osteopenia (low bone mass) is diagnosed, the physician may also discuss pharmacologic treatment and lifestyle changes to avoid disease progression. Osteopenia is considered a serious risk factor for developing osteoporosis and is common in people over 50. In addition, fractures may occur in people with osteopenia as well as those with osteoporosis, so close follow-up with a physician after diagnosis is important.
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National Osteoporosis Foundation, 1232 22nd Street N.W., Washington, D.C., 20037-1202, (202) 223-2226, (800) 231-4222, http://www.nof.org.
NIH Osteoporosis and Related Bone Diseases, National Resource Center, 2 AMS Circle, Bethesda, MD, 20892-3676, (202) 223-0344, (800) 624-BONE, (202) 466-4315, [email protected], http://www.niams.nih.gov/Health_Info/bone/default.asp.
Genevieve Slomski Ph.D.