Dual Energy X Ray Absorptiometry

Contents

1. What the test measures
2. Why ordinary X rays are not enough
3. Central and peripheral scans
4. On the table
5. Radiation
6. Reading the result
7. Where the method falls short
8. Repeating the scan
9. The next step in research

Running and reading DEXA bone density evaluations is part of the clinical experience of Candace Cooley, and the scan is the tool the wider field leans on whenever it counts weak bones. One recent figure shows why the count matters: in Britain, Medscape reports, hidden spinal fractures that no one has diagnosed are thought to affect about 2.2 million people each year, and the national guideline body now wants the same scanner to look for them.⁹

What the test measures

The full name describes the method: dual energy X ray absorptiometry, a measurement of how much X ray energy bone absorbs. MedlinePlus describes the scan as a low dose X ray that measures calcium and other minerals in bone; bones with more mineral are denser and tend to resist breaking.³ The National Institute of Arthritis and Musculoskeletal and Skin Diseases adds that the test usually measures the hip and spine because those are the weak bones that break most often.⁸ The abbreviation appears as both DEXA and DXA, and KFF Health News notes the second is pronounced “decks ah.”²

Why ordinary X rays are not enough

The NHS explains that unlike an ordinary X ray, a DEXA scan can detect very small reductions in bone density, which makes it possible to diagnose osteoporosis early, ahead of any fracture.⁴ The New York State Department of Health calls DXA of the hip and spine the preferred method of diagnosis and the only accurate way to find the disease while the skeleton is still intact.⁷ The NHS adds that doctors use the result to decide whether treatment for low density is needed, and that a total body version of the scan can also measure the amounts of bone, fat and muscle in the body, a use that is routine in children but reserved for research studies in adults.⁴

Central and peripheral scans

MedlinePlus separates two forms. A central scan measures the lower spine, hip, wrist or whole body and is usually done in a hospital or clinic. A peripheral scan uses a portable box to measure the finger, hand, forearm or foot in a few minutes and can be done in an office, a mobile van or a pharmacy, but gives less detail, so a worrying result is usually followed by a central scan.³ The Bone Health and Osteoporosis Foundation is firmer: peripheral DXA, peripheral CT and quantitative ultrasound give results that cannot be compared with a central measurement, serve mainly to pick out people who need one, and cannot diagnose osteoporosis or show whether a medicine is working.⁶

Figure

A central DEXA scan, step by step

1. 01
   PositionThe patient lies on her back on a padded table, usually in her own clothes.
2. 02
   ScanOne arm of the machine passes over the hips and spine while another passes beneath.
3. 03
   StillnessShe stays still, sometimes holding her breath, so the images do not blur.
4. 04
   ReportThe combined images go to a computer and a clinician reviews them.

On the table

The experience is brief. MedlinePlus puts a central scan at 10 to 30 minutes, with the patient motionless on a padded table while one arm of the machine passes over the hips and spine and another passes beneath.³ The Royal Osteoporosis Society notes that there is no tunnel and no injection, and that clothing with zips or underwired fastenings can spoil the pictures.⁵

Radiation

The dose is small. The Royal Osteoporosis Society compares it to about three days of ordinary background exposure.⁵ The Bone Health and Osteoporosis Foundation puts it at far less than a flight across the country.⁶ MedlinePlus still advises against the scan during pregnancy.³ The NHS makes the comparison the other way, noting that the dose is low enough that the health risk is much smaller than with standard X rays.⁴

Reading the result

The machine reports density as a comparison. For older adults the result is a T score, the gap between the measured density and a healthy young adult’s; for younger adults it is a Z score, which measures against peers instead.⁸ The Royal Osteoporosis Society stresses that density is only one part of bone strength, so the score is usually combined with a fracture risk calculation such as FRAX.⁵ MedlinePlus notes the general rule that a higher number means denser bone and a lower one a greater risk of fracture.³ The Bone Health and Osteoporosis Foundation adds that although the test is easy for the patient, the technology behind it is sophisticated, and it needs trained staff to perform and a qualified reader to interpret; it suggests asking whether the staff hold certification from a body such as the International Society for Clinical Densitometry.⁶

Where the method falls short

The NHS lists the limits. Arthritis of the spine or an old spinal fracture can distort a result, and the scan cannot tell whether low density comes from too little bone or from too little calcium in bone, as in osteomalacia.⁴ MedlinePlus adds that accuracy falls after spinal surgery or with spinal deformity.³ The Royal Osteoporosis Society notes that scans grow less accurate with age, which is one reason some people over 75 are offered treatment without one.⁵

Figure

Central DEXA and peripheral tests

	Central DEXA	Peripheral tests
Sites	Hip and lower spine	Wrist, finger, forearm, heel
Setting	Hospital or clinic	Office, van or pharmacy
Diagnosis	Used to diagnose osteoporosis	Screening only, results not comparable
Monitoring	Tracks treatment over time	Not useful for treatment response

Repeating the scan

A single scan shows density at one moment. The New York State guide explains that loss can only be shown by comparing a second scan, typically taken two years on, with the first.⁷ The Royal Osteoporosis Society recommends the same hospital and, where possible, the same machine, so results can be compared, and notes that a small decline between scans is expected in adults over 30.⁵ Whether and when another scan makes sense for one woman is a decision for her and her own clinician. Who should be offered a first scan at all is covered under osteoporosis screening.

Figure

How often a scan is repeated

high risk

2 years

moderate risk

3 to 5 years

low risk

10 to 15 years

The next step in research

The newest work asks the scanner to do more than measure. The Bone Health and Osteoporosis Foundation describes add ons available at some centers: a sideways image of the spine that finds compression fractures, a trabecular bone score that rates the internal structure of the vertebrae, imaging of the full thigh bone, and an analysis of hip shape.⁶ Medscape reports that Britain’s draft guidance of January 2026 would add vertebral fracture assessment to routine scans for adults 50 and older, since it takes only minutes on the same machine.⁹ Earlier, in 2011, NBC News quoted a researcher who wanted more study of the T score drop in the years around menopause, a gap that research has yet to close.¹ Methods without radiation, such as radiofrequency echographic multi spectrometry, are also under study, though none has replaced central DEXA for diagnosis.⁶ The background is in the entry on osteoporosis.