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Published by Bupa's health information team, September 2009.
This factsheet is for people considering having a DEXA scan to measure bone density. A DEXA scan can help look for signs of fragile bones and help assess your risk of developing fractures.
Your care may differ from what is described here and will be adapted to meet your needs. So it's important that you follow your doctor's advice.
About DEXA scan
A dual energy X-ray absorptiometry (DEXA) scan uses X-ray equipment and a computer to measure bone density.
A DEXA scan looks for signs of fragile bones (osteoporosis), which have a low density. The results of the scan help to estimate your risk of fractures in different bones, for example, your hip, wrist or bones of the spine. Doctors can also use DEXA scans to monitor how well treatment for osteoporosis is working.
The procedure is quick and painless. It involves exposing your body to a small dose of X-ray radiation.
DEXA scans done in hospital clinics are usually of your hip and lower spine. A scanning machine projects X-rays onto these bones. Some of this radiation travels straight through the bones and a certain amount is absorbed by them - how much depends on how dense the bones are. A detector measures how much radiation passes through your bones and sends the information to a computer. This works out a score of the average density for the bone and can compare it to expected measurements for someone of your age and sex. A low score indicates the bone is fragile and therefore more likely to fracture.
You're likely to benefit from having a DEXA scan if you have fractured a bone after only a minor fall or bump. For example, this could be a fall from less than the height of a chair. If you're at high risk of osteoporosis, you may also be advised to have a DEXA scan. High-risk groups include:
- women who had an early menopause (under 45) and didn't have hormone replacement therapy (HRT)
- women before the menopause who have had gaps of over a year between periods
- men or women who have a disease that leads to low bone density or causes bone loss, such as rheumatoid arthritis
- men and women under 65 who have taken steroid tablets for three months or more
- men or women with a mother who fractured her hip before she was 75
- women who have had an oophorectomy to remove their ovaries and no HRT to compensate for the menopause that this causes
If you have an eating disorder, you may have a DEXA scan to determine your bone density.
DEXA scans are routinely done as a hospital outpatient procedure in the imaging or radiology department.
What are the alternatives?
Alternative imaging tests such as ultrasound and computed tomography (CT) scans can be used to assess your fracture risk. However, these aren't usually done because they aren't as reliable as DEXA scans and also CT scans use larger doses of radiation than DEXA scans. Blood tests that claim to predict your risk of osteoporosis are available but experts advise that these aren't reliable.
Normal X-rays, also called plain X-rays, aren't helpful for diagnosing osteoporosis.
Preparing for your DEXA scan
You probably won't need to make any preparations before your scan. If you usually take medication, continue to take this as usual unless your doctor specifically tells you not to.
It's best to wear light clothing that doesn't have metal fasteners over your spine or tummy areas. Remove any metal jewellery that you wear in your navel because this can interfere with the scan of your spine.
A radiographer (a health professional trained to perform imaging procedures) will operate the scanning equipment.
A DEXA scan isn't recommended for pregnant women, unless there is an urgent medical reason. Please tell your radiographer if you could be pregnant.
You may be asked to sign a consent form. This confirms that you understand the benefits, risks and possible alternatives to the procedure and have given your permission for it to go ahead.
What happens during a DEXA scan
The scan usually takes about 10 minutes. You may be asked to remove your clothing and put on a gown.
You will be taken to the X-ray room and asked to lie down on an X-ray table. Your radiographer will operate the scanner from behind a window. He or she will be able to see, hear and speak to you during the procedure.
The X-ray machine will slowly pass over your body and send X-rays through the bones being tested, usually the hip and lower spine.
A detector will measure how much X-ray radiation passes through your bones and sends the information to a computer. It can take as little as 10 seconds to scan each bone - it's important to lie very still while this is happening.
What to expect afterwards
When you feel ready, you will be able to go home. You can drive if you wish.
A report will be sent to the doctor who requested your test. This can take a few days to arrive.
The results of the scan are given as a 'T-score'. This is a measure of how your bone density compares to the normal average for young, healthy adults whose bone density is at its peak. A T score of 0 means your bones are the same density as a young, healthy person of the same sex.
If your T score is:
- between 0 and -1 (for example, -0.5), your bone density is normal
- between -1 and -2.5, this is classed as osteopenia - this means your bone density is lower than normal, but you don't yet have osteoporosis
- below -2.5, this is classed as osteoporosis
You may sometimes be given a Z score when you have your scan. This is a measure of how your bone density compares to the average for a person of your own age. This score isn't used to diagnose osteoporosis, but can be used along with your age and gender to calculate your risk of having a hip fracture in the next 10 years.
What are the risks?
DEXA scans are commonly performed and safe. However, in order to make an informed decision and give your consent, you need to be aware of the possible side-effects and the risk of complications of this procedure.
You will be exposed to some X-ray radiation. At most, this is as much as with a chest X-ray - one of the lowest doses of radiation used in medical procedures. Although this is safe for adults, it can cause damage to an unborn child. Pregnant women are advised not to have a DEXA scan. If you are, or think you may be pregnant, please tell your doctor before your appointment. Your doctor will discuss alternatives to the procedure.
There are limitations to the information that can be obtained from DEXA scans. Many people do have fragile bones and have fractures, even though their T score doesn't indicate that they are at risk. This is why it's important that you and your doctor consider other risk factors when deciding whether to have treatment to help prevent fractures.
DEXA scan Q&As
See our answers to common questions about DEXA scan, including:
- Berger A. How does it work? Bone mineral density scans. BMJ 2002; 325:484
- Hain SF. DEXA scanning for osteoporosis. Clin Med 2006; 6(3):254-58
- Management of osteoporosis. Scottish Intercollegiate Guidelines Network (SIGN), June 2003. www.sign.ac.uk
- Osteoporosis. Clinical Knowledge Summaries. www.cks.library.nhs.uk, accessed 19 February 2009
- Osteoporosis. The Merck manual of health and aging. www.merck.com, accessed 21 February 2009
- Primary prevention. National Institute for Health and Clinical Excellence (NICE). www.nice.org.uk, October 2008
- Osteoporosis - secondary prevention including strontium ranelate. National Institute for Health and Clinical Excellence (NICE). October 2008. www.nice.org.uk
- Patient dose information. Health Protection Agency. www.hpa.org.uk, accessed 19 March 2009
- Scans and tests and osteoporosis. National Osteoporosis Society. www.nos.org.uk, accessed 18 February 2009
This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.
Publication date: September 2009