The DXA Bone Mineral Density Scan.
By: Adrian Rawlinson MD

The Bone Density test comes in many forms. You may have seen the small machines at Health Fairs or at your local pharmacy.

These devices measure bone density by ultrasound technology. It is inexpensive, portable and there is no radiation exposure. Several studies have shown it to be a good predictor of fracture risk. It is sometimes called Quantitative Ultrasound(QUS).

However, ultrasound is not recommended to monitor osteoporosis long term because of limited precision of the machine and the fact that bone mass at peripheral sites (e.g. heel) changes very slowly. If an ultrasound test reveals an abnormal result we suggest obtaining a confirmatory DXA scan of the hip and spine.

It is also possible to use Quantitative Computed Tomography (QCT) to measure bone density. It is used less often than DXA. It is more expensive, less reproducible and emits a higher radiation dose than DXA.

So let's talk a little about DXA. DXA is short for Dual X-Ray Absorptiometry. It is also known as DEXA, Bone Densitometry or the Bone Mineral Density (BMD) test/scan. It is really the 'gold standard' of bone density measurement techniques. DXA uses a very low radiation dose. The radiation is equivalent to the dose received when flying on an airplane from San Francisco to New York! So as you can see the exposure is minimal, but you should NOT undergo a scan if there is any chance of you being pregnant. DXA measures the BMD in one hip and in the lumbar spine. A few centers will measure both hips. Smaller, more mobile peripheral DXA devices are also used(pDXA). Talk to your physician about which Bone Density Test is available in your area.

So when you have had your scan you will be given a t-score. This is a statistical number that compares your bone density to that of a young adult. The more negative the t-score the worse your bone density. The lower the t-score the higher your risk of fracture. Take a look at this scale to see where your t-score lies.

The World Health Organisation classifies your t-score as follows:

T-SCORE better than -1.0 is NORMAL

T-SCORE from -1.0 to -2.5 is OSTEOPENIA

T-SCORE -2.5 or worse is OSTEOPOROSIS

On the day of the exam, eat normally, but don't take calcium supplements for at least 24 hours beforehand. Wear loose, comfortable clothing, avoiding garments that have zippers, belts, or buttons made of metal. Inform your physician if you recently had a barium examination or have been injected with a contrast material for a computed tomography (CT) scan or radioisotope scan; you may have to wait 10-14 days before undergoing a DEXA test. Women should always inform their physician or x-ray technologist if there is a possibility they are pregnant.

The results of a DEXA bone density exam are interpreted by a physician specially trained in Bone Densitometry.I am a Certified Clinical Densitometrist with the International Society for Clinical Densitometry (www.iscd.org). I send an interpretation of your results and a signed report to your primary care physician, who will work with you to develop a treatment plan. Usually available within a few days, your test results will be in the form of two scores:

T score — This number shows the amount of bone you have compared to a young adult of the same gender with peak bone mass. 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. It is used to estimate your risk of developing a fracture.

Z score — This number reflects the amount of bone you have compared to other people in your age group and of the same size and gender. If it is unusually high or low, it may indicate a need for further medical tests.

Despite its effectiveness as a method of measuring bone density, DEXA is of limited use in people with a spinal deformity or those who have had previous spinal surgery. The presence of vertebral compression fractures or osteoarthritis may interfere with the accuracy of the test. CT scans may be more useful in such instances. DEXA cannot predict who will experience a fracture, but can provide indications of relative risk.

Central DEXA devices are more sensitive than pDEXA devices, but they are also somewhat more expensive. The peripheral devices don't accurately follow changes in your bones during therapy. A test done on a peripheral location, such as your heel or wrist, may help predict the risk of fracture in your spine or hip. But because bone mass tends to vary from one location to the other, measuring the heel is not as accurate as measuring the spine or hip.

Adrian J Rawlinson MD
http://www.osteoporosis-advisor.com

About the Author

Dr. Rawlinson is the Medical Director of The Osteoporosis Center in San Francisco. He hails from Manchester, England and earned his medical degree from the University of Leeds, England in 1983.

He specializes in the assessment and management of non-surgical musculoskeletal and sports medicine related problems. He has a special interest in Osteoporosis and is a Certified Clinical Densitometrist.

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