Bone Density Scanning DXA
There is no simple way to look into your bones and see whether the inside structure has lost its strength so health professionals will use a range of scans, tests and checks to decide whether you have osteoporosis (or another bone condition) and whether you need a prescribed osteoporosis drug treatment.
Investigations and assessments used may include ordinary x-rays, blood tests, bone density scans, fracture risk assessment, magnetic resonance imaging (MRI) scans, radio isotope bone scans, computerised tomography (CT) scans and bone markers.
Which investigations you need will depend on your individual situation. The following section describes the scans and tests you might need.
There is also a procedure called a bone biopsy in which a piece of bone is extracted and looked at under the microscope. This could detect changes inside your bones but it is not a simple procedure so is rarely used to investigate osteoporosis.
Investigations for spinal fractures
I have back pain and lost height. What investigations do I need?
It may be that a bone or several bones in your spine have fractured (compressed) because they have lost their strength as a result of osteoporosis. Your doctor will need to examine you and discuss whether you have any signs or symptoms of compression fractures such as back pain, loss of height or curvature of the spine. Of course, other conditions such as osteoarthritis can cause these symptoms as well so your doctor will need to make a clinical judgement about whether your signs and symptoms are likely to have been caused by compression fractures.
Finding out whether you have fractures
If fractures are suspected, an ordinary x-ray is usually the best way to confirm whether fractures have occurred. Sometimes, your doctor will decide not to refer you for an x-ray if it isn’t going to change your management or treatment. This is to avoid exposing you to unnecessary radiation. However, being certain you have had a fracture is very useful in building up a picture of how strong your bones are. You may also feel that knowing you have had a fracture is important to you to help explain the pain you are experiencing. Discuss your concerns and questions with your doctor.
Understanding what’s caused your fractures
If you have had spinal fractures, your doctor will need to judge whether osteoporosis or something else has caused them and will want to know:
- Have you had an accident in the past that might have caused fractures? Obviously anyone will break a bone if sufficient force is experienced. If, however, a bone has broken after minimal trauma, your doctor will want to investigate further.
- Do you have another condition that causes fractures or that alters the shape of your bones such as Scheuermann’s disease (an arthritic condition identified by x-ray) or osteoarthritis?
Very occasionally, a type of cancer such as myeloma causes fractures. In this situation, you would usually have lost weight, be feeling very unwell and already have had investigations.
Blood tests are sometimes needed to see whether you have another medical condition that is causing your fractures.
A bone scan (using a radioisotope dye)
is occasionally used to check whether fractures are caused by other medical conditions or to pick up fractures that have been difficult to identify on x-ray. This involves having an injection of a very weak radioactive substance followed in a couple of hours by a scan to show up any recent fractures.
Sometimes fractures might be identified using an MRI scan, especially if hip or spinal fractures are not seen on a normal x-ray. MRI scans use a strong magnetic field to produce detailed pictures of soft tissue, ligaments and muscles. Often, MRI is used to check whether another problem is causing back pain such as soft tissue injuries or disc problems. MRI scans are more expensive than ordinary x-rays but the advantage is that they don’t use any radiation.
What tests and checks will tell me if my fractures are caused by osteoporosis?
If your fractures were the result of a simple fall and no other medical condition has caused them, it will be important that your doctor considers whether you have osteoporosis. A bone that breaks easily is an important sign of osteoporosis and research has shown that if you have had one fragility fracture, you are much more likely to have another in the future.
'Osteoporosis', meaning bones are fragile, may be identified using many different scans and tests but there is specific ‘diagnosis’ that is based on the results of a bone density scan.
Osteoporosis is diagnosed using a bone density scanning machine when the amount of bone you have (bone density) as measured on the scan is found to be significantly lower than average. Research has shown that bone fragility can be assessed most accurately by considering all your risk factors (see Fracture Risk Assessment below) including your bone density.
Bone density scanning
A bone density scan using a densitometry x-ray (DXA) measures how much ‘bone mineral’ is in the area being measured—usually one hip and the lower part of your spine. Research has shown that the lower your bone density is, the greater your risk is of having a fracture. The scan results are commonly given as a ‘standard deviation’ (SD) - the number of units above or below average. If your bone density is 2.5 SD below average, this is described as ‘osteoporosis’. If your bone density is between the lower end of the normal range and the ‘osteoporosis’ range, you are said to have ‘osteopenia’.
Osteoporosis measured on a bone density scan is a risk factor for fracture in the same way that high cholesterol is a risk factor for a heart attack.
If a bone density scan explained exactly what was happening inside your bones and how strong they were, scanning could be used routinely to see who had and who hadn’t ‘got osteoporosis’, who was likely to have a fracture and who needed an osteoporosis drug treatment.
However, the situation is more complicated than this. Your bone density scan result isn’t a perfect measure of your bone strength and doesn’t assess the ‘quality’ of your bone tissue; only the ‘quantity’.
The scan doesn’t show whether the structure inside your bones has broken down, which is what often makes bones lose their strength. Research has now proved that bone fragility can be assessed more accurately by a fracture risk assessment, which combines the results of your bone density scan with other proven risk factors for fracture.
Does a bone density scan identify fractures?
A bone density scan might identify a spinal fracture but only if the fracture is in the area being scanned. However, it is not an effective method for diagnosing fractures. There is a new scanning technique called vertebral fracture assessment (VFA), sometimes used alongside the usual types of DXA scan, which can help to see whether you have had a spinal fracture.
Fracture risk assessment
Although low bone density on a scan increases your risk of fracture, many other factors will also contribute to your risk.
When doctors are deciding who needs a drug treatment to reduce their risk of a fracture, they use a fracture risk assessment rather than simply diagnosing osteoporosis on a scan. Drug treatments, which have been proved to rapidly reduce the risk of fractures occurring, are then prescribed for those people with a high risk of fracture.
A number of computer-based questionnaires called ‘fracture risk assessment tools’ have been developed to help health professionals identify who is at high risk of fracture. A commonly used assessment tool is called FRAX. If a health professional is using one of these assessment tools, you will be asked a number of questions about your risk factors including your age, sex, weight, height, history of previous fractures, parental history of hip fractures and whether you have other medical conditions known to affect bone strength.
One of the main benefits of these tools is that they provide some numbers to quantify your risk of having a fracture. Using the answers you provide, this is usually expressed as a probability (in percent) of you breaking a bone in the next 10 years. Health professionals can then use available guidelines to decide whether your risk is high enough for you to need an osteoporosis drug treatment.
Other investigations to see why you might have low bone density
If your bone density is unusually low, your doctor may refer you to a hospital specialist for further investigations, particularly if you are under 50 years of age when bone density is usually stable. Blood tests and other investigations may be appropriate to see whether you have another medical condition that is causing low bone density and osteoporosis such as coeliac disease, hyperparathyroidism, hyperthyroidism or hyperprolactinaemia (high levels of a hormone called prolactin).Treating these medical conditions may help to improve your bone strength. Sometimes, low bone density is a sign of another bone condition such as osteomalacia (soft bones caused by severe vitamin D deficiency). You may need to be referred to a specialist, such as an endocrinologist, at a hospital for investigations or treatment of these other conditions.
What does a bone density scan tell me about my bone strength?
Osteoporosis is also a term used to describe low bone density as measured on a bone density (DXA) scan. It means your bones may have lost strength. A DXA scan measures the amount or quantity of bone tissue in the area measured but not the quality of the bone; having an ‘osteoporosis result’ doesn’t always mean the struts inside your bones have broken down, although it’s more likely. In combination with other risk factors such as being older (over 50 years) and having had fragility fractures already, low bone density can be used to predict your risk of fractures in the future. This can help with decisions about whether you need an osteoporosis drug treatment.
Investigations after you break a bone
I have broken a bone after a simple fall. What investigations will I need?
Often, the first indication of osteoporosis is when a bone breaks easily—this might be a wrist in your fifties or sixties—after a simple fall from a standing height. Of course, anyone will break a bone with sufficient force but if you break a bone very easily, then you may have osteoporosis. Hopefully, a health professional will have discussed osteoporosis and your bone health with you when you went to hospital with your fracture. Some hospitals and GP practices have a Fracture Liaison Service with specialist health professionals—usually specialist nurses—following you up and ensuring you have a bone check or fracture risk assessment.
Again, ordinary x-rays will be used to identify the fracture. As with a spinal fracture, you may need other tests and investigations to make sure other medical conditions haven’t caused your fracture. If you are over 50 years of age, it would be good practice for your doctor to consider a fracture risk assessment after a fracture occurs because breaking a bone can be a very important sign of osteoporosis and fragile bones. You may be referred for a bone density scan as part of this assessment.
I think I am at risk. What investigations do I need?
If you are under 40 or a pre-menopausal woman, the current recommendation is to discuss your risk factors with your doctor who will decide whether further investigations are necessary. If you have broken a bone easily, especially in your spine or in your hip, wrist or forearm, or you are taking high-dose glucocorticoids (steroids), a bone density scan may be recommended to help measure your bone strength.
If you are over 40 and, especially, if, as a woman, you are post-menopausal, research has shown that a fracture risk assessment is the best approach to predicting your risk of breaking bones in the next 10 years. This is because you are reaching an age when fractures are more likely and an osteoporosis drug treatment may be needed. Fracture risk assessment is currently considered the best way of working out who needs an osteoporosis drug. Having a bone density scan will often form part of the fracture risk assessment.
Investigations for children and younger people
Is the diagnosis of osteoporosis any different for people under 50 and for children?
All the changes inside the bone that happen with osteoporosis are less common in younger people and especially in children. Interpreting bone density scans and assessing fracture risk in this setting is more complex, particularly because of changes that occur in the skeleton during growth and puberty. It has been suggested that even with a very low bone density result, a diagnosis of osteoporosis should only be made if you have fractures as well as low bone density compared to your own age group. A referral to a hospital specialist is usually helpful in this situation.
Computerised tomography (CT) scanning
This type of scanning uses x-rays and a computer to take pictures of bone and put images together. CT scanners can be used to measure bone density and may be useful following compression fractures in the spine when getting an accurate result on a bone density scan can be difficult. However, CT scanning uses higher levels of radiation than DXA scanning so a DXA scan is usually considered preferable.
Some specialist centres may carry out ‘bone marker’ tests.
When the cells that break down and build up bone are at work, they leave behind chemical traces. These are called bone markers and can be measured in urine or blood. The results of the tests can be used to monitor the rate of bone loss in people with osteoporosis. They can tell whether you are losing bone too fast and they may be used to monitor your response to drug treatments. These tests cannot be used alone to measure bone density or to assess your risk of breaking bones.
Ultrasound can be used to examine structures inside the body. Sound waves of extremely high frequency, inaudible to the human ear, are beamed into the body. The echoes of reflected sound are used to build up an electronic image or measurement of the structure being examined. Ultrasound scanning has proved useful in visualising many different parts of the body. Portable and relatively inexpensive ultrasound machines have been developed. These are designed to look at bone structure and strength in a different way from bone density scanning and usually examine the heel, wrist or finger. Ultrasound scanning can certainly tell you something about your bone strength and risk of fracture, especially your risk of hip fracture if you are older (75 years or over) but it is not as well developed as other methods such as DXA. If you have a low ultrasound result, you may need to be referred for a DXA scan, especially if you have other risk factors.
How are fractures or broken bones diagnosed?
Normal x-rays are used to see if a hip, wrist or other bone has broken. Height loss and spinal curvature are not always caused by osteoporotic fractures so a normal x-ray can be useful for identifying other problems such as disc disease.
Are there other types of bone density scans?
DXA scans are currently considered the ‘gold standard’ and are the only type of scan generally recommended. However, other types of scans can be useful in some situations. There are a number of other techniques available including:
-peripheral DXA (pDXA) machines
-QCT (quantitative computed tomography)
Do I need to see a specialist?
Many people with osteoporosis or at high risk of fracture can be managed by doctors, nurses and other practitioners at your GP practice. If a referral is made for bone density scanning (which usually takes place in a hospital), then an interpretation of the scan should be sent to your doctor so that they know what it means and what possible treatments are recommended. If your medical situation is complicated or the GP is uncertain about any aspect of your care, you may be referred to a hospital specialist and/or to see the clinical nurse specialist in osteoporosis. This is important with more unusual types of osteoporosis.
You cannot insist on a hospital referral because this is a clinical decision made by your GP but you can make a request and discuss your questions and reasons. In the same way, you cannot insist on a particular medical treatment although, in most practices, you can arrange to see another doctor to discuss your situation.