Bone Mineral Density Tests

A bone mineral denisity (BMD) test is used in screening for osteoporosis to test the strength and solidness of the bones.

Osteoporosis (or porous bone) is a bone disease in which bones become weak and are more likely to break.

Without prevention or treatment, osteoporosis can progress without pain or symptoms until a bone breaks (fractures).

• Fractures commonly occur in the hip, spine, and wrist.
• Osteoporosis is often the underlying cause of bone fractures.

Osteoporosis is not just an "old woman's disease." Although it is more common in white or Asian women older than 50 years of age, osteoporosis can occur in almost any person at any age. In fact, more than 2 million American men have osteoporosis, and in women, bone loss can begin as early as 25 years of age.

The image on the left shows decreased bone density in osteoporosis. The image on the right shows normal bone density.

Arrow indicates vertebral fractures.

Building strong bones and reaching peak bone density (maximum strength and solidness), especially before the age of 30, can be the best defense against developing osteoporosis. Also, a healthy lifestyle can keep bones strong, especially for people older than 30 years of age.

A. Normal spine, B. Moderately osteoporotic spine, C. Severely osteoporotic spine.

Osteoporosis is more or less preventable for most people. Prevention is very important because, while treatments for osteoporosis are in place, currently no cure exists. Prevention of osteoporosis involves several aspects, including nutrition, exercise, lifestyle, and, most importantly, early screening with bone density tests.

The Importance of Screening for Osteoporosis

Early detection of low bone mass (osteopenia) or osteoporosis is the most important step for prevention and treatment. If osteopenia or osteoporosis has occurred, a person can take action to stop the progression of bone loss.

Remember, effective treatment or prevention cannot take place if a person does not know he or she has, or is at risk for, osteoporosis.

• The only way to accurately test the strength and solidness of the bones is with bone mineral density (BMD) tests. Bone mineral density tests measure the solidness and mass (bone density) in the lumbar spine, hip, and/or wrist, which are the most common sites of fractures due to osteoporosis.
• Other tests measure bone density in the heel or hand. These tests are performed like X-rays. They are painless, noninvasive, and safe. The risk of radiation is very minimal, much less than even having a chest X-ray film.

Risk Factors for Osteoporosis

Certain factors are associated with an increased risk of developing osteoporosis (see Prevention of Osteoporosis). Take a one-minute osteoporosis risk test from the International Osteoporosis Foundation.

If a person has any of these risk factors or other signs of osteoporosis, a doctor may recommend that bone mass is measured. Risk factors for osteoporosis include the following:

• Advancing age
• Early menopause (age <45 years)
• Female sex
• Asian or white race
• Family history of hip fracture
• Low body weight
• Long-term corticosteroid therapy
• Chronic disorders associated with osteoporosis, such as anorexia nervosa or liver disease
• Previous broken bones with minimal trauma
• Poor diet without enough calcium and vitamin D
• Lack of exercise
• Smoking

Current Recommendations

According to current recommendations in the United States by the National Osteoporosis Foundation, the following individuals should have a bone mineral density (BMD) test:

• All women 65 years and older, regardless of risk factors, to screen for postmenopausal osteoporosis
• Younger postmenopausal and premenopausal women who have one or more risk factors (other than being white, postmenopausal, and female)
• Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity)
• Men aged 70 and older
• Younger men who have broken a bone or who have one or more risk factors
• Adults taking medication that is associated with bone loss, such as prednisone or methylprednisolone (Medrol)
• Anyone being considered for treatment with prescription medication to strengthen bone
• Anyone taking prescription medication to strengthen bone (to monitor treatment effect)

Medicare and Bone Mineral Density Testing

• Medicare covers bone mineral density (BMD) testing for the following individuals 65 years and older:
  • Women with low estrogen levels who have risk factors for osteoporosis
  • Men and women with abnormalities of the spine (vertebral abnormalities)
  • Men and women who are receiving (or are going to receive) long-term steroid (glucocorticoid) therapy
  • Individuals with primary hyperparathyroidism (excess of parathyroid hormone)
  • Men and women on drug therapy for osteoporosis who are being monitored to determine the effectiveness of the drug therapy
• Medicare permits individuals to repeat bone mineral density testing once every 24 months.

BMD tests measure the solidness and mass (bone density) in the spine, hip, and/or wrist (the most common sites of fractures due to osteoporosis). Some bone mineral density tests measure bone in the heel or hand.

These measurements are performed like X-ray films, and they are the only reliable way to determine loss of bone mass. They are painless, noninvasive, and safe.

Doctors examine bone mineral density test results to do the following:

• Determine the extent of bone loss (osteopenia or osteoporosis) before a fracture occurs
• Confirm a diagnosis of osteoporosis if a person already has broken bones (fractures)
• Predict the chance of a person having a fracture in the future
• Determine the rate of bone loss and monitor the effects of treatment (follow-up tests performed to monitor treatment are usually conducted every two years)

Several tests are available to assess bone density. Densitometry means the measurement of bone density. Densitometry tests are not painful, and they are completely noninvasive, which means no surgery. Central machines measure density in the hip, lumbar spine, and total body. Peripheral machines measure density in the finger, wrist, kneecap, shinbone, and heel. The most common types of tests are listed below:

• Dual energy X-ray absorptiometry (DXA or DEXA) scanning is a special low-radiation X-ray that can detect bone loss -- even very small amounts of bone loss. DXA scans are the most commonly used method of bone mineral density measurement. They are used to measure lumbar spine, forearm, and hip bone densities. Peripheral dual energy X-ray absorptiometry (pDXA) measures the bone density in the forearm, finger, and heel. Single-energy X-ray absorptiometry (SXA) measures the bone density in the wrist or heel.
• Quantitative computed tomography (QCT) scanning measures the bones of the lower (lumbar) spine because these bones change as a person ages. The peripheral QCT (pQCT) scan measures the forearm bone density.
• Quantitative ultrasound (QUS) uses sound waves to measure bone density at the heel, shin, and finger.
• Radiographic absorptiometry (RA) scanning uses an X-ray film of the hand and a small metal wedge to calculate bone density.

Bone mineral density tests measure bone density in the spine, hip, and/or wrist, which are the most common sites of fractures due to osteoporosis. The results of the bone mineral density test are compared to two standards (norms):

• The age-matched reading, known as the Z-score, compares a person's bone density to what is expected in someone of equivalent age, sex, and size. However, among older and elderly adults, low bone mineral density is common, so comparison with age-matched norms can be misleading.
• The young-normal reading, known as the T-score, compares bone density to the optimal peak bone density of a healthy young adult (30 years old) of the same sex. The T-score determines fracture risk, which increases as bone mineral density falls below young-normal levels. The T-score, which is a comparison between the solidness (density) of the bones and the bones of the average young healthy population, is measured in standard deviations (SDs).
• SD is a statistical term that describes variation in a population. According to the World Health Organization's (WHO) diagnostic categories, individuals whose T-score is within 1 SD of the norm are considered to have normal bone density. Scores below the norm are indicated in negative numbers.
• For most bone mineral density tests, -1 SD equals a 10%-12% decrease in bone density. The risk for broken bones increases by 50%-100% for every SD below the young-normal standard.

In addition to calculating Z-scores and T-scores, the DXA report may include a FRAX (fracture risk assessment tool) score. FRAX uses the bone density measurement, and other risk factors, to calculate a person's risk of breaking any major bone or hip due to osteoporosis in the next 10 years. FRAX is used for those people who have not yet taken bone-building medication.

If the probability of a hip fracture in the next 10 years is calculated to be more than 3%, or the probability of a fracture of any major bone due to osteoporosis is greater than 20%, treatment with prescription medication to reduce the risk of bone fracture is often recommended.

WHO Definitions of Osteoporosis Based on Bone Density Levels

• Normal: Bone density is within 1 SD (+1 or -1) of the young adult mean.
• Low bone mass (osteopenia): Bone density is 1 to 2.5 SDs below the young adult mean (-1 to -2.5 SD).
• Osteoporosis: Bone density is 2.5 SDs or more below the young adult mean (less than -2.5 SD).
• Severe (established) osteoporosis: Bone density is more than 2.5 SDs below the young adult mean and one or more broken bones (osteoporotic fractures) has occurred.

With the information from a bone density test, a doctor can identify the degree of bone loss and determine whether a person is at risk for fracture. In general, the lower the bone density (weaker bones), the higher the risk for fracture. Test results help determine which prevention or treatment options are right.