A bone mineral density (BMD) test measures the mineral density (such as calcium) in your bones using a special X-ray, computed tomography or CT scan, or ultrasound. From this information, an estimate of the strength of your bones can be made. Calcium is constantly being added to and taken away from bone. When calcium is taken away faster than it is added, the bones become lighter, less dense, and more porous. This makes the bones weaker and increases their risk of fracture.
Loss of bone mass occurs as part of the natural process of aging. Bones naturally become thinner (called osteopenia) as you grow older, because existing bone is broken down faster than new bone is made. As this occurs, the bones lose minerals, heaviness (mass), and structure, making them weaker. With further bone loss, osteopenia develops into osteoporosis.
The thicker your bones are, the longer it takes to develop osteoporosis. Although osteoporosis can occur in men, it is most common in women older than age 65. Regular X-rays cannot detect mild bone loss. A bone must lose at least a quarter of its weight before a regular X-ray can detect the problem.
Bones to Test
Controversy exists over which bones are best to use for BMD measurements. The bones most commonly used are those in the lower spine and hip. These bones generally have the greatest amount of bone loss and are at the highest risk of fracture. In special cases, bones in the wrist may be used. Ultrasound testing is done on the bone in the heel.
If your bone density is lower than normal, you can take steps to increase your bone strength and reduce your risk of fracture. Some ways to increase bone density and strength include taking calcium and vitamin D supplements, doing weight-bearing exercise (such as walking), weight training (such as lifting weights or using weight machines), and using medications such as calcitonin (Miacalcin), alendronate (Fosamax), risedronate (Actonel), raloxifene (Evista), teriparatide (Forteo), or hormone replacement therapy (usually for women past menopause).
Bone mineral density is measured to:
Detect the presence of osteopenia if you have risk factors for developing osteoporosis.
Detect osteoporosis. Signs of osteoporosis include an unexpected fracture, loss of height, and bone changes seen on regular X-ray pictures.
Monitor treatment for osteoporosis.
Bone density of women who are older than age 65 for osteoporosis. Women younger than 65 may be screened for osteoporosis if they have certain risk factors that indicate screening may be beneficial.
BMD Test Procedure
A bone mineral density scan is usually done in the hospital radiology department by a technologist. Peripheral dual-energy X-ray absorptiometry (P-DEXA) machines are portable units that can be used in a doctor’s office.You will need to lie on your back on a padded table. You can usually leave your clothes on. You may need to lie with your legs straight or with your lower legs resting on a platform built into the table. The machine will scan your bones and measure the amount of radiation they absorb. The DEXA technique, which scans the hip and lower spine, takes only about 20 minutes to perform. Other techniques may take 30 to 45 minutes.
A bone mineral density (BMD) test measures the mineral density (such as calcium) in your bones using a special X-ray, computed tomography (CT) scan, or ultrasound. Results of bone mineral density tests can be reported in several ways. Your T-score is your BMD compared to the average score of a healthy 30-year-old. It is expressed as a standard deviation (SD), which is a statistical measure of how closely each person in a group is to the average (mean) of the group.
The average BMD is determined by measuring the bone density of a large group of healthy 30-year-olds (young adult reference range). BMD values are then reported as a standard deviation from the mean of this reference group. About 90% of people have a BMD value within 2 standard deviations of this mean.
A negative () value indicates that you have thinner bones (lower bone density) than an average 30-year-old. The more negative the number is, the less bone density you have compared with an average 30-year-old.
A positive (+) value indicates that you have higher bone density than an average 30-year-old.