Bone Density Scan

  • Medical Author:
    Catherine Burt Driver, MD

    Catherine Burt Driver, MD, is board certified in internal medicine and rheumatology by the American Board of Internal Medicine. Dr. Driver is a member of the American College of Rheumatology. She currently is in active practice in the field of rheumatology in Mission Viejo, Calif., where she is a partner in Mission Internal Medical Group.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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What is bone mineral density (BMD)?

The absolute amount of bone as measured by bone mineral density (BMD) testing generally correlates with bone strength and its ability to bear weight. The BMD is measured with a dual energy X-ray absorptiometry test (referred to as a DXA scan). By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke.

It is important to remember that BMD testing cannot predict the certainty of developing a fracture. It can only predict risk. It is also important to note that a bone density scan, or test, should not be confused with a bone scan, which is a nuclear medicine test in which a radioactive tracer is injected that is used to detect tumors, cancer, fractures, and infections in the bone.

The World Health Organization has developed definitions for low bone mass (osteopenia) and osteoporosis. These definitions are based on a T-score. The T-score is a measure of how dense a patient's bone is compared to a normal, healthy 30-year-old adult.

Normal: A bone BMD is considered normal if the T-score is within 1 standard deviation of the normal young adult value. Thus a T-score between 0 and -1 is considered a normal result. A T-score below -1 is considered an abnormal result.

Low bone mass (medically termed osteopenia): A BMD defines osteopenia as a T-score between -1 and -2.5. This signifies an increased fracture risk but does not meet the criteria for osteoporosis.

Osteoporosis: A BMD greater than 2.5 standard deviations from the normal (T score less than or equal to -2.5) defines osteoporosis.

Based on the above criteria, it is estimated that 40% of all postmenopausal Caucasian women have osteopenia and that an additional 7% have osteoporosis.

Why is BMD measurement important?

Determining a person's BMD helps a doctor decide if a person is at increased risk for osteoporosis-related fracture. The purpose of BMD testing is to help predict the risk of future fracture so that the treatment program can be optimized. The information from a BMD is used to aid a decision as to whether nonprescription and/or prescription medicine therapy is needed to help reduce the risk of fracture. Additionally, if a patient has a fracture or is planning orthopedic surgery, a diagnosis of osteoporosis might affect the surgical plan. A fracture that could potentially heal in a cast with normal bone mass might require either a longer period of casting or even surgery if the patient has osteoporosis.

Medically Reviewed by a Doctor on 6/2/2015
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