Patient education: Bone density testing (Beyond the Basics)
- Elaine W Yu, MD
Elaine W Yu, MD
- Assistant Professor of Medicine
- Harvard Medical School
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 United States, osteoporosis leads to approximately 2.0 million fractures, including :
●550,000 fractures of the vertebrae, the bones in the spine
●300,000 hip fractures
●400,000 wrist fractures
●810,000 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 that 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. Health care 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 education: Osteoporosis prevention and treatment (Beyond the Basics)".)
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, health care 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 health care providers sometimes recommend testing in men or women younger than 65 years 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:
●Long-term use of steroid (glucocorticoid) medications such as prednisone
●Low body weight (less than 127 lbs or 58 kg)
●History of a nontraumatic 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 — 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 the best at predicting who will have an osteoporotic fracture, at identifying who should be treated for osteoporosis, and at monitoring response to treatment.
If you are unable to lie on an examination table, it will not be possible to measure your spine and hip bone density. Instead, you can sit beside the DXA machine for a scan of your forearm. When the hip and spine cannot be measured, the diagnosis of osteoporosis can be made using a DXA measurement of the forearm. If you have a condition known as hyperparathyroidism or have an overactive thyroid (hyperthyroidism), the forearm should also be measured (in addition to the spine and hip) because the bone density at the forearm may be lower than at the hip with these conditions.
If you have a DXA study done, make sure that your doctor gets all the DXA measurements, not just a summary statement. These measurements can hold important clues that are not always on the summary statements.
If your doctor recommends a follow-up DXA (usually two years or more between studies), try to have the follow-up study done at the same facility as the first one. There are different models of DXA instruments, and the bone density measurements are easier to compare if they have been taken on the same model.
Quantitative computerized tomography — This is a type of computed tomography (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 dual-energy x-ray absorptiometry (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 5 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.
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 your 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 weightbearing exercise. (See "Patient education: Osteoporosis prevention and treatment (Beyond the Basics)".)
Low bone mass (osteopenia) — Low bone mass (osteopenia) is the term health care 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.
If 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 education: Osteoporosis prevention and treatment (Beyond the Basics)".)
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, you can take osteoporosis medications combined with calcium and vitamin D supplements, and you can do an exercise program. (See "Patient education: Calcium and vitamin D for bone health (Beyond the Basics)" and "Patient education: Osteoporosis prevention and treatment (Beyond the Basics)".)
Note that if you have previously had a low trauma bone fracture, you may also be identified as having osteoporosis and need to take osteoporosis medications, regardless of your bone density T-score.
Fracture prediction tool — Fracture Risk Assessment Tool (FRAX) is an online tool that was developed to predict your 10-year likelihood of having a minimal trauma fracture. You can use it to determine your fracture risk even if you have not had a dual-energy x-ray absorptiometry (DXA) test, but you will get a more accurate prediction if you include DXA results. If you decide to use the FRAX tool on your own, without a DXA study, and the results indicate a high risk of fracture, it is good to also have a DXA test. This DXA will serve as a baseline by which your doctor can follow your response to treatment.
The FRAX tool should only be used the first time you and/or your doctor decide that it is time to evaluate your bone health and risk of fracture. FRAX should not be used if you have already had a minimal trauma fracture or are already on treatment aimed at preserving your bone health and preventing future fractures.
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 initial bone density results and whether you have risk factors that represent an ongoing threat to your bones.
●If initial bone density testing shows you have a T-score of -2.00 to -2.49 at any site, or 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 two 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.
●In women 65 years of age and older at baseline screening, with a T-score of -1.50 to -1.99 at any site, and with no risk factors for accelerated bone loss, we will typically perform a follow-up dual-energy x-ray absorptiometry (DXA) in three to five years.
●In women 65 years of age and older at baseline screening, with normal or slightly low bone mass (T-score -1.01 to -1.49), and with no risk factors for accelerated bone loss, we will typically perform a follow-up DXA in 10 to 15 years.
Repeat bone density measurements may be most valuable for patients who are taking a medication to treat osteoporosis to determine the efficacy of treatment and for patients who are not being treated but have medical conditions that can cause bone loss to determine if they need treatment.
Bone density tests help health care providers spot bone loss in people who might otherwise have no symptoms. The tests are painless, quick, and safe, and they 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 health care provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed on our website (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for health care professionals, are also available. Some of the most relevant are listed below.
Patient level information — UpToDate offers two types of patient education materials.
The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.
Patient education: Bone density testing (The Basics)
Patient education: Osteoporosis (The Basics)
Patient education: Calcium and vitamin D for bone health (The Basics)
Patient education: Vitamin D deficiency (The Basics)
Patient education: Primary hyperparathyroidism (The Basics)
Patient education: Hip fracture (The Basics)
Patient education: Vertebral compression fracture (The Basics)
Patient education: Cadmium toxicity (Cadmium poisoning) (The Basics)
Patient education: Medicines for osteoporosis (The Basics)
Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.
Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.
The following organizations also provide reliable health information.
●National Library of Medicine
(www.nlm.nih.gov/medlineplus/ency/article/007197.htm, available in Spanish)
●Osteoporosis and Related Bone Diseases National Resource Center
●National Osteoporosis Foundation
●The Fracture Risk Assessment Tool
●Osteoporosis Society of Canada
●Hormone Health Network
(www.hormone.org, available in English and Spanish)
- Burge R, Dawson-Hughes B, Solomon DH, et al. Incidence and economic burden of osteoporosis-related fractures in the United States, 2005-2025. J Bone Miner Res 2007; 22:465.
- 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.
- National Osteoporosis Foundation. Clinician's Guide to Prevention and Treatment of Osteoporosis. http://www.nof.org/professionals/clinical-guidelines (Accessed on April 28, 2008).
All topics are updated as new information becomes available. Our peer review process typically takes one to six weeks depending on the issue.