What Is a Bone Density Test?
A bone mineral density test (BMD), a non-invasive and painless test, is the best way to determine your
bone health. Dual energy x-ray absorptiometry (DEXA) is the best current test to measure BMD. The test is quick and painless; it is similar to having an x-ray taken, but uses much less radiation.
- Hip
- Spine
- Wrist
- Finger
- Shin bone
- Heel.
How Does This Test for Bone Density Work?
To measure bone mass, the person lies on a flat padded table and remains motionless while the "arm" of the instrument passes over the whole body or over selected areas. While the measurement is performed, a beam of low-dose x-rays from below the table passes through the area being measured. These x-rays are detected by a device in the instrument's arm. The bone densitometry machine converts the information received by the detector into an image of the skeleton and analyzes the quantity of bone the skeleton contains. The results are usually reported as BMD, or "bone mineral density," the amount of bone per unit of skeletal area.
For the spine measurement, the person's lower legs rest on a Styrofoam cube with the hips flexed. For the hip measurement, the toes are placed in a "pigeon-toed" position to rotate the hips and provide the largest projected area to measure. For the arm measurement, the person sits on a chair beside the machine and places an arm into a holding device while the measurement is taken. For a total body measurement, which provides individual measurements of the legs, trunk, pelvis, ribs, arms, and skull, the person simply lies flat and motionless.
It is important to remember this warning: If you move while the measurement is taking place, errors can occur. Usually it is not too difficult for the person to remain motionless because, using the latest equipment, each measurement requires less than a minute to perform.
How a Bone Density Test Is Scored
Your
bone density test is compared to two norms: "young normal" and "age-matched." Young normal, known as your T-score, compares your bone mineral density to optimal or peak density of a 30-year old, healthy adult and determines your fracture risk, which increases as bone mineral density falls below young-normal levels. Age-matched, known as your Z-score, compares your bone mineral density to what is expected in someone your age and body size. Among older adults, however, low bone mineral density is common, so comparison with age-matched norms can be misleading.
The difference between your bone mineral density and that of a healthy, young adult is referred to as a standard deviation (SD).
What Do the Results Mean?
As outlined in the World Health Organization's diagnostic categories, individuals whose T-score on a test for bone density is within one standard deviation of the "norm" are considered to have normal bone density. Scores below the "norm" are indicated in negative numbers. For example, a score from -1 to -2.5 SD below the norm indicates low bone mass, or
osteopenia, and a score of more than -2.5 SD below the norm is considered a diagnosis of
osteoporosis. For most BMD tests, -1 SD equals a 10 to 12 percent decrease in bone density.
Who Should Get a Bone Density Test?
A bone density test is recommended for those people:
- With one or more additional risk factors for osteoporotic fracture (besides menopause)
- Who have had a fracture (broken bone); this test can help determine if osteoporosis was the underlying cause
- Who are age 65 and older
- Who are considering therapy for osteoporosis (if BMD testing will help make the decision)
- Who have been on hormone replacement therapy for a long time.
Possible Risks When Testing Bone Density
The technology used for the test for
bone density is quite safe. Radiation exposure is exceedingly small -- about the equivalent of playing outdoors all day on a summer day or flying across the United States in a jet plane. With this test, a person receives less than 10 percent of the radiation received in a standard chest x-ray. State regulatory agencies permit routine measurement of healthy children.
A bone density test is not painful. There is no needle stick involved, and one cannot feel anything when the x-rays pass through the body. The hardest part is remaining still for the minute or so when the measurement is performed. The testing instrument does not enclose you.
