In short
A DEXA scan reports a T-score and often a Z-score. The thresholds that separate normal bone from low bone mass and osteoporosis are fixed numbers, and which score applies to you depends on your age.
Emergency if: Severe pain and inability to bear weight after a fall, or back pain with leg weakness, numbness around the groin or loss of bladder control.
On your report
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01
What the scan measures
A bone density scan uses a low dose of X-rays to measure how much mineral is packed into a given area of bone, usually at the hip and the lower spine. Those sites are chosen because they are where fractures cause the most harm and because they predict fracture risk elsewhere reasonably well.
The radiation dose is small, the scan takes a few minutes, and nothing is injected. You may be offered one 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.
02
The T-score thresholds
The T-score compares your bone density with that of a healthy young adult. If your T-score is minus 1 or higher, your bone is healthy. Between minus 1 and minus 2.5, you have osteopenia, a less severe form of low bone mineral density than osteoporosis. At minus 2.5 or lower, you might have osteoporosis.
Each whole point down the scale represents a substantial difference in bone strength, so the gap between minus 1.2 and minus 2.4 is not trivial even though both sit in the same band. It is also normal for the hip and spine to give different scores; the lower of the two generally drives the diagnosis.
03
When the Z-score is the one that matters
The Z-score compares your bone density with the average for healthy people of your age, ethnicity and sex rather than with a young adult. If your Z-score is minus 2.0 or less, your bone mineral density is low for your age group.
Which score applies depends on who you are. T-scores are used for postmenopausal women and men aged 50 and over. Z-scores are used for premenopausal women, men younger than 50, and children — because comparing a 30-year-old with a young-adult peak is uninformative, while being well below your own age group suggests a specific cause worth hunting for.
04
What the number does not decide on its own
Treatment decisions are not made from the T-score alone. Most fragility fractures actually occur in people with osteopenia rather than osteoporosis, simply because far more people sit in that band — so a score of minus 1.8 in someone with several other risk factors may warrant more action than minus 2.6 in someone with none.
Risk calculators combine the density result with age, previous fracture, family history, smoking, alcohol, steroid use and rheumatoid arthritis to estimate the actual probability of fracture over the next ten years. A previous fracture from a minor fall is itself a strong signal and can lead to treatment whatever the scan shows.
05
Repeat scans and what helps in between
Bone changes slowly, so repeating the scan too soon mostly measures the machine rather than your bones. Intervals are typically a couple of years or longer, and comparisons are most reliable when done on the same scanner. A small change between scans is often not a real change at all.
Between scans, the measures that genuinely support bone are adequate calcium and vitamin D, weight-bearing and resistance exercise, stopping smoking, moderating alcohol, and reducing the risk of falling — reviewing medicines that cause dizziness, checking vision, and sorting out hazards at home. Preventing the fall matters as much as strengthening the bone.
When to act
Routine — see a doctor
A T-score in the osteopenia band with no fractures: review calcium, vitamin D, exercise and fall risk, and agree a repeat interval with your doctor.
Same-day — call promptly
New persistent back pain with loss of height in someone with low bone density, which can indicate a spinal compression fracture.
Emergency — act now
Severe pain and inability to bear weight after a fall, or back pain with leg weakness, numbness around the groin or loss of bladder control.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- Bone Mineral Density Tests: What the Numbers Mean — NIAMSniams.nih.gov
- DEXA Scan — NHSnhs.uk
- Osteoporosis — NIAMSniams.nih.gov
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