DEXA Scans Explained

Your clinician has ordered a bone density test, or a friend has mentioned a “DEXA scan” and you’re wondering whether you need one. At Humanic Blue, we unpack what the scan measures and what its result can—and cannot—tell you.

Despite the alphabet soup, the test itself is straightforward. DXA—the professional abbreviation, although DEXA is common in searches—stands for dual-energy X-ray absorptiometry. It uses a very small dose of X-rays to measure bone mineral density—the amount of mineral in an area of bone—usually at the lower spine and hips. RadiologyInfo describes the scan as painless and usually lasting 10 to 30 minutes. The result helps a clinician estimate the risk of a broken bone; it does not predict exactly who will break one.

Preparation, repeat timing and coverage depend on why the test was ordered. Those details follow below.

What should you know before the scan?

According to RadiologyInfo.org, the patient site reviewed by the Radiological Society of North America, American College of Radiology and American Society of Radiologic Technologists, the test requires little preparation.

• Eat normally on the day of the test.
• Ask the imaging site about supplements. RadiologyInfo’s general preparation guidance says to avoid calcium supplements for at least 24 hours before DXA; the site performing your scan can confirm its instructions for you.
• Wear clothes without metal, such as zippers, belts or buttons, or expect to change into a gown.
• Mention recent barium or contrast tests. The facility may ask you to wait 10 to 14 days after a barium examination or injected contrast for a CT or radioisotope scan.
• Tell the technologist if you might be pregnant.

During the scan

You lie on a padded table while a scanning arm passes over the hip and lower spine. RadiologyInfo describes the test as painless and reports a usual duration of 10 to 30 minutes, depending on the equipment and body areas examined.

The radiation dose is very low. RadiologyInfo’s radiation dose table, reviewed in 2025, lists adult bone densitometry at approximately 0.001 millisieverts (mSv), a unit used to compare radiation exposure. That is comparable to three hours of average natural background radiation. It lists screening digital mammography at 0.28 mSv. These are approximate effective doses for an average-sized adult, not an exact dose for every scan.

Some centers also offer vertebral fracture assessment (VFA), a low-dose side-view image made on the same machine to look for breaks in the spinal bones that may not have caused recognized symptoms. It adds only a few minutes. The International Society for Clinical Densitometry (ISCD) positions base VFA use on combinations of T-score, age, height loss, a previous spinal fracture and use of glucocorticoid steroid medicines rather than offering it automatically to everyone.

What does the result tell you after the scan?

A T-score compares bone mineral density (BMD) with a healthy young-adult reference. A Z-score compares it with people of the same age and sex, using population-specific reference data. These are comparison scores, not percentages of bone lost.

The ISCD prefers T-scores for postmenopausal women and men age 50 or older, and Z-scores for women before menopause and men younger than 50. In men younger than 50, BMD alone cannot diagnose osteoporosis. The usual T-score categories are:

T-score	Category
−1.0 or higher	Normal
Below −1.0 but above −2.5	Low bone mass (osteopenia)
−2.5 or lower	Osteoporosis

What those categories mean for fracture risk and treatment is covered in Osteoporosis vs. Osteopenia. A clinician often combines the result with FRAX, a tool that estimates the chance of a fracture over the next ten years using age and other risk factors.

How does the choice of T-score or Z-score change the comparison in a DXA report?

How often might a DXA scan be repeated?

There’s no single interval for everyone. The ISCD says repeat testing should have a defined purpose, be likely to affect management and be individualized by age, starting bone density, treatment and factors that can cause rapid change. The ISCD lists situations where shorter intervals can be appropriate. These include medicines that can speed bone loss—glucocorticoids such as steroid medicines, aromatase inhibitors that lower estrogen, and androgen-deprivation treatment that lowers testosterone—as well as bone-building medication. Other examples are poor nutrient absorption, major inflammatory disease, prolonged immobility, weight-loss surgery or menopause caused by surgery. For population screening, the 2025 USPSTF review found that repeating BMD within four to eight years did not improve fracture-prediction accuracy in available cohort studies; lower baseline T-scores and older age were associated with faster transition to osteoporosis. Monitoring treatment is a different question.

Small changes can reflect measurement variation. The facility’s least significant change is the minimum difference between scans that exceeds its expected measurement variation. That threshold helps distinguish a likely real change from measurement noise, as covered in Can Bone Density Improve?.

Screening start ages are covered in When Should Women Get a Bone Density Test?. The USPSTF recommends screening women 65 and older and younger postmenopausal women at increased risk; it finds the evidence insufficient to recommend routine screening in men.

What distinguishes a meaningful density change from the limits of measurement precision?

Coverage and cost

Medicare Part B covers bone-mass measurements once every 24 months—or more often when medically necessary—for people who meet listed conditions. Those include women whose clinician determines they are estrogen-deficient and at risk, people whose X-rays suggest osteoporosis, low bone mass or vertebral fractures, people taking or planning steroid treatment, people with primary hyperparathyroidism (overactive parathyroid glands) and people being monitored on osteoporosis medication. The covered test costs nothing when the provider agrees to accept Medicare’s approved payment amount (called accepting assignment), but Medicare advises checking coverage and possible costs when a service is recommended more often than the benefit allows.

What else a DXA machine can measure

The same technology can estimate body composition, separating lean soft tissue (muscle and other non-fat soft tissue), fat mass and bone mineral content. Research studies use it this way, such as the SWAN study of body composition across menopause covered in Protect Muscle During Menopause. A body-composition scan is a different use from a clinical bone-density examination. Medicare’s public bone-mass benefit page describes testing for fracture risk and does not list elective body-composition analysis as part of that benefit; verify coverage separately rather than assuming it is included.