DXA bone density scan: what it shows

What is a DXA scan?

DXA, also written DEXA, stands for dual-energy X-ray absorptiometry, a scan that measures bone density. The scan is used to help diagnose osteoporosis, a condition in which bones become weaker. The NHS describes a DEXA scan as a test that uses low-dose X-rays.

Who is offered one?

NHS trust pages say a doctor can refer a person for a DEXA scan if they may be at risk of osteoporosis, and the criteria differ between services. One NHS trust referral form lists being a current smoker and having had a previous low-trauma fracture among the risk factors it asks about. A doctor usually makes the referral. Results are combined with a fracture risk assessment rather than used alone.

Why might a doctor suggest it?

A bone density scan measures bone density and helps to judge the risk of a fracture. Unlike an ordinary X-ray, it can detect small reductions in bone density, so osteoporosis can be found before a bone breaks. The NHS lists reasons for a scan that include breaking bones easily, long-term use of steroid medicines, and early menopause.

What happens during the scan?

The person lies on their back on a flat table. A scanning arm passes slowly over the body, usually the hip and lower spine. NHS pages describe the scan as quick and painless, and the time it takes varies between services.

How much radiation is involved?

The dose is low. The NHS says a DEXA scan uses a much lower level of radiation than a standard X-ray, similar to what a person is exposed to naturally over about three days.

Who should not have one?

The NHS says the scan uses a low dose of radiation and the risks are much lower than with standard X-rays. Pregnancy is the main caution: the NHS says bone density scans and X-rays are not recommended in pregnancy. The person lies on their back on a flat, open X-ray table while a scanning arm passes slowly over them. An NHS trust page says no special preparation is needed, although metal fasteners on clothing may need to be removed.

What do the T-score and Z-score mean?

The T-score compares a person's bone density with that of a healthy young adult of the same sex. Scores are grouped into bands described as normal, low bone density (osteopenia) and osteoporosis. Osteoporosis is diagnosed using the lowest T-score across the lumbar spine, the neck of the thigh bone and the total hip. The Z-score compares a person with others of the same age, and it is used mainly for premenopausal women, men under 50 and children.

Does the result decide treatment?

A T-score does not decide treatment alone. UK osteoporosis guidance uses a fracture risk tool such as FRAX, which estimates the chance of a broken bone over ten years from bone density and other risk factors, so bone density alone does not decide treatment.