In short
A DEXA scan measures the mineral in your bones and reports a T-score and a Z-score. A T-score of -2.5 or lower defines osteoporosis. The number is one part of fracture risk, alongside age, previous fractures and steroid use.
A DEXA scan measures how much mineral your bones contain, and reports it as two numbers, the T-score and Z-score. The T-score is the one that defines osteoporosis. A value of -2.5 or lower on that scale is the definition, and anything from -1.0 upwards is considered normal.
The number matters, but it is not the whole answer. Bone density is one input into the question everybody actually cares about, which is how likely you are to break a bone. Age, a previous fracture, long courses of steroid tablets and several other factors change that risk independently of what the scan shows.
This article explains what the machine measures, how to read both scores, what the report cannot tell you, who should be scanned, and what usually follows a low result.
What a DEXA scan measures
DEXA stands for dual energy X-ray absorptiometry. Two X-ray beams of different energies are passed through the body, and detectors measure how much of each is absorbed. Because bone and soft tissue absorb them differently, the machine can work out how much mineral is present in the bone alone (NHS).
The result is a bone mineral density figure, reported for each site scanned. It is a low-dose test using much less radiation than a standard X-ray, which is why the radiographer can stay in the room with you. It is not recommended in pregnancy, as with any X-ray.
The scan itself is undemanding. You lie on a padded table, usually with your clothes on, and a scanning arm passes over your hips and spine while you stay still. The whole thing takes between ten and thirty minutes (MedlinePlus).
A word about the small portable machines that measure the heel, wrist or finger. These peripheral devices give useful information about fracture risk, but they cannot give the detail of a central bone density test of the hip and spine, and a low reading on one is normally a reason to arrange the full scan rather than a diagnosis in itself.
Reading the T-score
The T-score compares your bone density with that of a healthy young adult of the same sex. It is expressed in standard deviations, which is why it is a small number with a minus sign rather than a percentage.
MedlinePlus sets out the bands used worldwide. A T-score of -1.0 or higher is normal bone density. A T-score between -1.0 and -2.4 means low bone density, known as osteopenia, and a raised chance of going on to develop osteoporosis. A T-score of -2.5 or lower meets the definition of osteoporosis.
Two points are worth holding on to. The scale is not linear in its consequences: each step down represents a substantial increase in the chance of a fracture, so the difference between -2.0 and -3.0 is larger than it looks. And the T-score is designed for postmenopausal women and for men aged 50 and over. In anyone younger it is the wrong comparison to use.
When the Z-score is the one that matters
The Z-score compares your bone density with other people of your own age, sex, weight and ethnic background. It is the score used for premenopausal women, for men under 50 and for children.
That difference of comparison group changes what the number means. A low T-score in a seventy-year-old is partly the ordinary arithmetic of ageing. A low Z-score at thirty-five is not, because it says your bones are thinner than they should be for someone your age, and that almost always has a reason worth finding.
In practice a markedly low Z-score triggers a search for a secondary cause: an overactive thyroid or parathyroid, coeliac disease or another malabsorption problem, chronic kidney or liver disease, low testosterone in men, early menopause, an eating disorder, or a course of steroid tablets that ran on longer than anyone intended. That search is often the most valuable thing the scan produces.
Why the hip and the spine are scanned
These two sites are chosen because fractures there carry the greatest consequences, and because they respond to treatment in measurable ways. The lower of the two scores usually drives the osteoporosis diagnosis, so a normal spine alongside a hip in the osteoporosis band is still osteoporosis.
The forearm is scanned in a few specific situations, such as overactive parathyroid disease or when body size makes the hip and spine unreadable. It is not a routine part of a DEXA scan and is not a substitute for either of the main sites.
The spine has a quirk that catches people out. Anything that adds dense material to the spinal column pushes the reading upward and makes bones look stronger than they are. Arthritis in the spine, previous collapsed vertebrae, spinal surgery, scoliosis and calcification in the aorta all do this (MedlinePlus). When the spine is unreliable for one of those reasons, the hip reading carries the diagnosis instead.
This is also why a reported spine score that is much higher than the hip score deserves a second look rather than reassurance. It is often the arthritis talking.
What the report cannot tell you
A bone density test measures quantity, not quality. Bone strength also depends on its internal architecture and on how it is turning over, and no routine scan measures those directly.
It also cannot tell you why the density is low. The scan is the start of that question, not the answer, which is why blood tests usually follow a low result.
And it cannot tell you that you will or will not break a bone. Osteoporosis is not usually painful until a bone breaks, and plenty of fractures occur in people whose T-score sat in the osteopenia band (NHS). Waiting for a scan to fall below -2.5 before taking bone health seriously is a common and avoidable error.
Fracture risk is more than a number
The NHS is explicit that the results of a bone density scan are used alongside a fracture risk assessment, using tools such as FRAX or Q-Fracture, rather than in isolation.
These tools combine the density result with the things that raise risk independently of it.
- Age, sex, and a low body mass index
- A previous fracture as an adult, or a hip fracture in a parent
- Steroid tablets taken for more than three months
- Smoking, heavy drinking, and little regular exercise
- Inflammatory conditions such as rheumatoid arthritis, and hormone or malabsorption problems
The practical consequence is that two people with identical scores can need quite different advice. Someone with osteopenia, a wrist fracture last year and three years of steroid tablets behind them may face a higher risk than someone with a worse score and none of that history. If you are on or starting long-term steroids, what steroids do over time is worth reading alongside this.
Who should be scanned, and when
NIAMS reports the screening advice used in the United States: bone density screening for women over 65, and for women of any age who have factors that increase the chance of developing osteoporosis. For men, the task force did not make a recommendation, because the evidence was not there (NIAMS).
The NHS frames it by risk rather than age alone: a scan may be needed if you are over 50 with a risk of developing osteoporosis, or under 50 with other risk factors such as smoking or a previous broken bone.
In rheumatology practice a DEXA scan is also arranged for a set of specific situations: before or soon after starting a long course of steroid tablets, in inflammatory arthritis and other autoimmune conditions that affect bone, after an early menopause or removal of the ovaries, after a fracture from a minor fall, where height has dropped noticeably, and in long-standing coeliac disease or other malabsorption.
What happens after a low result
The first step is looking for a treatable cause, which usually means blood tests: calcium, phosphate, vitamin D, kidney and liver function, thyroid function, and further tests guided by what turns up. Correcting a vitamin D deficiency matters here, because bone-strengthening treatment works poorly when vitamin D is low.
The second is the part that is entirely in your hands. Weight-bearing and resistance exercise, enough dietary calcium, stopping smoking, keeping alcohol modest, and making the house safer to walk around in all change outcomes. Whether you need a supplement as well as food is a separate question, covered in who actually needs calcium and vitamin D supplements.
The third is medicine. Bone-strengthening treatments genuinely reduce fractures, and they come in several forms, some taken as tablets and some given as an injection or an infusion in a day-care setting (procedures at the clinic). Which one suits you depends on the score, the fracture history, kidney function and what else you take.
One caution that matters more than most people realise: some of these treatments should not simply be stopped when a course ends, because stopping without a plan can undo the benefit quickly. Always agree the next step with the doctor who prescribed it rather than letting a prescription lapse.
How often the scan should be repeated
Repeating too soon is the commonest waste. Real change in bone density is slow, and the difference between two scans done close together is usually measurement noise rather than anything real.
MedlinePlus gives the intervals in common use: roughly every two years for people at high risk, every three to five years at moderate risk, and every ten to fifteen years at low risk. Your own interval depends on whether treatment has started and how high the risk was to begin with.
One practical detail makes a genuine difference. Wherever possible, have the repeat scan on the same machine at the same centre. Differences between machines and between software versions can be larger than the change you are trying to detect, and comparing two centres' reports often produces a false alarm or false reassurance.
Preparing for the scan and the appointment
You may be asked to stop calcium supplements for a day or two before the scan, because tablets in the gut can be picked up by the machine. Wear clothes without metal buttons, zips or buckles, and leave jewellery at home. Tell the department if you are or might be pregnant, and if you have had a barium study, a CT with contrast or a nuclear medicine scan in the past week, since those can interfere with the reading.
For the appointment afterwards, bring the full report rather than the summary line. The numbers, the sites scanned and the machine used are all needed, and a conclusion of "osteopenia" on its own cannot be compared with anything later. Bring every previous DEXA report you have, your current height, and a complete list of medicines and supplements including any steroid courses, however long ago.
Sudden severe back pain in someone with thin bones, or an inability to stand or bear weight after a fall, needs assessment straight away rather than a routine appointment. Those are the two ways a fracture announces itself, and both are treatable. What to expect at a first visit covers the rest of the preparation.
Common questions
What T-score means I have osteoporosis?
A T-score of minus 2.5 or lower meets the definition of osteoporosis. Between minus 1.0 and minus 2.4 is called low bone density, or osteopenia. Minus 1.0 or higher is normal. The T-score is the right comparison only for men over 50 and women past the menopause.
Is a DEXA scan safe, and does it hurt?
It is painless and uses a much lower dose of radiation than a standard X-ray, low enough that the radiographer can remain in the room with you. You lie still on a padded table for roughly ten to thirty minutes with your clothes on. It is avoided in pregnancy, as with any X-ray.
How often should a bone density scan be repeated?
Common intervals are about every two years at high risk, three to five years at moderate risk, and ten to fifteen years at low risk. Bone changes slowly, so repeating sooner usually measures noise. Where possible use the same machine and centre, because machines differ.
My spine score is much better than my hip score. Why?
Anything dense in the spinal column raises the reading. Spinal arthritis, previously collapsed vertebrae, surgery, scoliosis and calcification in the main artery all do it, making bones look stronger than they are. When that happens the hip reading is treated as the more reliable one.
Sources
- MedlinePlus - Bone density scan medlineplus.gov
- NHS - Bone density scan (DEXA scan) www.nhs.uk
- NIAMS - Osteoporosis: diagnosis, treatment and steps to take www.niams.nih.gov
- NHS - Osteoporosis www.nhs.uk
This article is general information and does not replace a consultation.