Bone Density Scan (cont.)
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.
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.
In this Article
- Bone density scan facts
- What is osteoporosis?
- How does osteoporosis occur?
- What is bone mineral density (BMD)?
- What information is on a DXA report?
- Why is BMD measurement important?
- What is the relationship between BMD and fracture risk?
- Who should have BMD testing?
- How is BMD measured?
- What are other methods of measuring BMD?
- How often should DXA scans be repeated to monitor treatment?
- What is the cost of DXA?
- What about the accuracy of BMD testing in the doctor's office using smaller equipment?
- Find a local Rheumatologist in your town
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 more 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.
What information is on a DXA report?
There is some variation in DXA reports depending on the facility performing the test. All reports should include the following:
- The date of the test, location, and equipment used for the test (manufacturer and model of the densitometer)
- The reason the test was performed
- The overall diagnosis (normal bone density, osteopenia, or osteoporosis) based on the results of the scan
- It should mention the results of the test at each site tested. The hip and lumbar spine are always tested. Many facilities also measure bone density at the forearm. The bone density is usually reported with three different numbers. First, the actual bone density is reported. This is measured in grams per centimeter squared (g/cm2). Because the exact bone density varies based on the manufacturer and model of the densitometer, the bone density is also reported as a T-score and a Z-score. The T-score is a measure of how dense a patient's bone is compared to a normal, healthy 30-year-old adult. The Z-score is a measure of how dense a patient's bone is compared to the average person of the same age and gender.
- Comparison of the bone density to any prior tests performed at the same facility
- Many reports include a calculation estimate of the patient's risk of bone fracture based on the results of the bone density scan. This is reported as the risk over the following 10 years of breaking a bone.
- A notation suggesting how long before a follow-up test is needed
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. Sometimes spinal surgeons treat patients with low bone density with bone building medication prior to surgery in order to improve the surgical outcome of bone that is operated on.
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