Bone Mineral Density Testing: Helpful information about DEXA Scanning

Rose Chon Health Guide
  • Bone density scanning, also called dual energy x-ray absorptiometry (DXA or DEXA) or bone densitometry, is the most widely used method to measure bone mineral density and is the only method that can make a definitive diagnosis of osteoporosis and monitor a patient’s response to treatment. It is an enhanced form of x-ray technology that requires no injections, sedation, special diet or any other advance preparation and the test is performed by a physician or technician in about 30 minutes.

     

    DEXA is most often performed on the lower spine and hips. Portable DEXA devices, including some that use ultrasound waves rather than x-rays, measure the wrist, fingers or heel and are sometimes used for screening purposes.

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    There is some controversy over which bones are best to use for bone density measurements. DEXA is most often performed on the lower spine and hip, which are the bones most commonly used during screening exams. In special cases, bones in the wrist, fingers or heel are sometimes used for screening purposes. For such cases, portable DEXA devices, including some that use ultrasound waves rather than x-rays can be used.

     

    How does the procedure work?

    The machine sends a thin, invisible beam of low-dose x-rays with two distinct energy peaks through the bones being examined. One peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is the patient’s bone mineral density.

     

    How is the procedure performed?

    First, the patient lies on a padded table and usually can keep their clothes on for the procedure. An x-ray generator is located below the patient and an imaging device is positioned above.

     

    To assess the spine, the patient’s legs are supported on a padded box to flatten the pelvis and lower spine. To assess the hip, the patient’s foot is placed in a brace that rotates the hip inward. In both cases, the imaging device is slowly passed over the area, generating images on a computer.

     

    How are the results reported and what do they mean?

    You test results can be reported in the form of two scores called a “T score” and a “Z score.”

     

    The T score is used to estimate your risk of developing a fracture. It shows the amount of bone you have compared with a young adult of the same gender with peak bone mass. 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 of the group. A score above -1 is considered normal. A score between -1 and -2.5 is classified as osteopenia, the first stage of bone loss. A score below -2.5 is defined as osteoporosis.

     

    The following table contains the World Health Organization’s definitions of osteoporosis based on T scores.

     

    Bone Mineral Density

     

    T-score

    Normal:

    2.5 to 1 below the young adult reference range (2.5 to -1)

    Low bone mass (osteopenia):

    1 to 2.5 SDs below the young adult reference range (-1 to -2.5)

    Osteoporosis:

    2.5 or more SDs below the young adult reference range (-2.5 or less)

    Severe osteoporosis:

    2.5 or more SDs below the young adult reference range (-2.5 or less) and the presence of one or more bone fractures


  • The Z score indicates that amount of bone you have compared with other people in your age group and of the same size and gender. A negative value indicates that you have thinner bones and weaker bones than the average of the people in your group. The more negative the number is, the less bone density you have compared with others in your group. A positive value means you have higher bone density and stronger bones than the average of the people in your group.

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    Who should have a DEXA scan?

    • Women over 65 years old, regardless of risk factors
    • Postmenopausal women with one or more risk factors for osteoporosis
    • Women who have been on hormone replacement therapy for prolonged periods
    • All postmenopausal women who have had a broken bone

     

    How much radiation exposure is there with a DEXA scan?

    The amount of radiation a patient is exposed to is quite small.  It is approximately 1/30th that of a chest x-ray, which is comparable to the naturally occurring radiation you are exposed to in one week.

     

    Will my insurance cover the cost of a DEXA scan?

    A DEXA scan costs approximately $300. Many insurance companies cover the cost, but check with your insurance provider for the specific coverage you receive. Medicare began coverage of screenings in 1998. A repeat screening is covered once every 24 months. Medicare will also pay for a DEXA in specific situations other than screening, including:

    • Patients who have been noted to have “thin bones” or certain fractures on a conventional bone x-ray.
    • Patients who are taking more than 7.5 mg of prednisone per day.
    • Patients with an overactive parathyroid gland.
    • Patients already taking an FDA approved drug for the treatment of osteoporosis to see how well it is working.

     

    Ultimately, if you are interested in the procedure, you should discuss the DEXA scan in further detail with your doctor to determine if it is right for you.
Published On: April 10, 2007