Osteoporosis — thin, fragile bones — affects an estimated 200 million people worldwide and causes more than 8.9 million fractures per year globally. Yet it produces no symptoms until a bone breaks. A DEXA scan (dual-energy X-ray absorptiometry) is the standard test for measuring bone mineral density before that point.
Here's how the test works, what the results mean, and what a low score actually implies for your health.
How a DEXA scan works
A DEXA machine passes two low-energy X-ray beams through the body — typically targeting the lumbar spine and hip. Because bone and soft tissue absorb X-rays differently, the machine can calculate the mineral content of bone at those sites. The whole scan takes around 10–20 minutes, uses very low radiation (less than a day's background exposure), and requires no injections or preparation.
T-score: comparing you to a young adult
The T-score compares your bone density to the average peak bone density of a young healthy adult of the same sex. This is the primary score used to diagnose osteoporosis.
- **T-score above −1.0:** Normal bone density
- **T-score −1.0 to −2.5:** Osteopenia (low bone density, below normal but not osteoporosis)
- **T-score below −2.5:** Osteoporosis
- **T-score below −2.5 with a prior fragility fracture:** Severe osteoporosis
Z-score: comparing you to your peers
The Z-score compares your density to the average for your age, sex and ethnicity. A Z-score below −2.0 in a premenopausal woman, a man under 50 or a child is considered abnormal and prompts investigation for a secondary cause — such as vitamin D deficiency, coeliac disease, long-term steroid use or hyperthyroidism.
Who should be screened
- Women aged 65 and above (routine screening)
- Men aged 70 and above
- Postmenopausal women under 65 with risk factors: family history, low body weight, smoking, prior fracture
- Anyone who has had a fragility fracture (a fracture from a fall from standing height)
- Long-term users of corticosteroids, certain anticonvulsants or aromatase inhibitors
What a low score means in practice
Osteopenia does not mean fracture is inevitable. It means bone density is lower than average and that protecting bone health through calcium, vitamin D, weight-bearing exercise and lifestyle changes (stopping smoking, limiting alcohol) is important. Many people with osteopenia never progress to osteoporosis.
Osteoporosis typically prompts a FRAX score calculation — a tool that combines bone density with other risk factors to estimate the 10-year probability of a major fracture. The result, not the T-score alone, guides whether medication is recommended.