A bone density test is the only way to find thinning bones before one breaks. The most common version, the DXA (or DEXA) scan, is quick, painless and uses very little radiation. The report, however, can be confusing: T-scores, Z-scores, FRAX percentages and several measurement sites. This guide explains how the scan works, how to prepare, who should have one and how to make sense of the numbers.
What a DXA scan is
DXA stands for dual-energy X-ray absorptiometry. The machine sends two low-dose X-ray beams with different energy levels through your body. Bone and soft tissue absorb each beam differently, and by comparing the two the scanner calculates how much mineral is packed into a given area of bone. The result is your bone mineral density, or BMD, reported in grams per square centimeter.
A standard, or central, DXA measures the lower spine and the hip. These sites matter most because spine and hip fractures carry the greatest consequences, and BMD at the hip is the strongest predictor of hip fracture. When the hip or spine cannot be measured reliably, for example because of joint replacements, severe arthritis or a body size beyond the table limit, the forearm may be scanned instead.
Is it safe? Radiation and comfort
A DXA uses a small fraction of the radiation of most X-ray exams. According to RadiologyInfo, a resource from two major radiology societies, the dose is less than one-tenth of a standard chest X-ray and less than the natural background radiation you absorb in a single day from your surroundings. The Bone Health & Osteoporosis Foundation compares it favorably with a cross-country flight.
The scan is painless. There are no needles, no injections and no enclosed tube. The main thing you will notice is that you need to lie still. If you are or might be pregnant, tell your clinician and the technologist beforehand.
How to prepare and what to expect
Before the day
- Skip calcium supplements for at least 24 hours before the scan, as advised by MedlinePlus and RadiologyInfo. Undissolved tablets in the gut can interfere with the reading. Eat normally otherwise.
- Mention recent imaging. If you have had a barium study or a contrast injection for a CT or nuclear medicine scan, you may need to wait about 10 to 14 days.
- Bring your referral and, if possible, results from any previous scan. Comparisons are most accurate on the same machine, or at least the same type of machine.
- Wear comfortable clothes without metal zippers, buttons, belts or underwire near the hips and lower back. You may be asked to change into a gown and remove jewelry, keys and eyeglasses.
During the scan
You lie on your back on a padded table. For the spine, your legs rest on a cushioned block to flatten your lower back. For the hip, your foot is placed in a brace that turns the hip slightly inward. A scanner arm passes slowly overhead. A central scan usually takes about 10 to 30 minutes from start to finish. Some centers also take a quick side image of the spine, called a vertebral fracture assessment, to look for hidden compression fractures.
Look for a center whose staff are certified in bone densitometry, for example by the International Society for Clinical Densitometry (ISCD). Careful positioning makes a real difference to accuracy.
How to read your T-score
For postmenopausal women and men aged 50 and older, the key number is the T-score. It shows how far your bone density sits above or below the average for a healthy young adult of the same sex at peak bone mass. Each whole number is one standard deviation. A negative score means your bones are less dense than that young-adult reference.
| T-score | Category | What it generally means |
|---|---|---|
| -1.0 or higher | Normal | Bone density in the expected healthy range |
| Between -1.0 and -2.5 | Low bone mass (osteopenia) | Thinner than ideal; fracture risk depends heavily on other factors |
| -2.5 or lower | Osteoporosis | Bone density low enough to diagnose osteoporosis |
Your report will usually list several sites, typically the lumbar spine (L1–L4), the total hip and the femoral neck (the narrow upper part of the thighbone). The diagnosis is normally based on the lowest of these scores, not the average.
Example
Suppose a report shows a spine T-score of -1.4, a total hip score of -1.8 and a femoral neck score of -2.6. Even though two sites are in the osteopenia range, the lowest score, -2.6 at the femoral neck, falls in the osteoporosis range, so that is the category the reading clinician would usually report. This is an illustration only; your own clinician interprets your scan.
A T-score is a measure of density, not a direct prediction of whether you will break a bone. Two people with the same score can have very different fracture risks depending on age, prior fractures and other factors. That is where FRAX comes in.
When the Z-score is used instead
The Z-score compares your bone density with the average for people of your own age and sex (and, in some reports, ethnicity). NIAMS and the ISCD advise using Z-scores rather than T-scores for:
- Premenopausal women
- Men younger than 50
- Children and teenagers
Comparing a young person with a young-adult reference makes little sense, so the age-matched comparison is more meaningful. Under ISCD positions, a Z-score of -2.0 or lower is described as below the expected range for age; above -2.0 is within the expected range. A low Z-score at any age can point to an underlying cause of bone loss, such as a hormone problem, a digestive condition or a medicine, and usually prompts further tests. Osteoporosis in these groups is not diagnosed on bone density alone.
FRAX: your 10-year fracture probability
FRAX is a widely used risk calculator. It combines clinical risk factors, including age, sex, weight, height, previous fracture, a parent's hip fracture, smoking, alcohol intake, glucocorticoid use and rheumatoid arthritis, with or without your femoral neck BMD. It produces two numbers:
- Your 10-year probability of a hip fracture
- Your 10-year probability of a major osteoporotic fracture (hip, spine, forearm or shoulder)
Many DXA reports now include FRAX results automatically. They are especially useful for people with osteopenia, because they help a clinician decide whether medicine is worthwhile. Treatment thresholds differ between countries and guidelines, so ask your clinician what your numbers mean for you rather than comparing them with figures found online.
Who should be tested and how often
The U.S. Preventive Services Task Force (2025) recommends screening for:
- All women aged 65 and older
- Postmenopausal women younger than 65 who are at increased fracture risk based on a clinical risk assessment
It found insufficient evidence to make a recommendation for or against routine screening in men. The Bone Health & Osteoporosis Foundation also suggests testing for men 70 and older, women 50 to 64 and men 50 to 69 with risk factors, and anyone who breaks a bone after age 50. People taking long-term steroid medicines or with conditions that affect bone may be tested at other ages.
How often?
There is no one-size-fits-all interval. Testing is generally repeated when a result could change your care. A common pattern is a repeat scan one to two years after starting or changing treatment, or one to two years later if you are untreated but close to a treatment threshold. For people whose first scan is normal and who have few risk factors, the USPSTF review noted that progression to osteoporosis tends to be slow, so the gap between tests can be much longer. Your clinician will set a schedule based on your results.
Peripheral tests and ultrasound
You may see quick bone checks offered at health fairs, pharmacies or mobile vans. These measure the heel, wrist or finger using peripheral DXA, peripheral CT or quantitative ultrasound. Ultrasound uses no radiation at all.
These tests can flag people who might benefit from a full scan, but they cannot be used to diagnose osteoporosis by the WHO criteria or to track treatment. If a peripheral test suggests low bone density, follow up with a central DXA of the hip and spine.
Cost and insurance
Prices for a DXA vary widely by country, region and facility, and depend on whether the scan is covered. In the United States, many private insurance plans cover screening for people who meet guideline criteria, often without cost sharing; check with your plan before booking.
For people with Medicare, Part B covers a bone mass measurement once every 24 months, or more often when medically necessary, if you meet at least one condition. These include being an estrogen-deficient woman at risk for osteoporosis, having X-rays that suggest osteoporosis, osteopenia or spine fractures, taking or planning to take prednisone-type steroids, having primary hyperparathyroidism, or being monitored on osteoporosis medicine. According to Medicare.gov, you generally pay nothing for the test if your provider accepts assignment.
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What to do after your results
- Normal: keep up bone-friendly habits, including enough calcium and vitamin D, regular weight-bearing and strength exercise, not smoking and moderate alcohol. Ask when, or whether, to retest.
- Osteopenia: ask for your FRAX result and whether medicine is recommended. Lifestyle steps and falls prevention matter a great deal here.
- Osteoporosis: discuss treatment options, possible causes and a follow-up plan. Our osteoporosis guide explains the main medicine groups in plain language.
- Low Z-score: expect further tests to look for an underlying cause.
Whatever your result, keep a copy of the report and bring it to future scans so changes can be tracked accurately. You can check your intake with the calcium calculator, review your risk factors with the bone health risk check, and read about safe training in exercises for strong bones. If you need a referral or a scan, the find care page can point you toward a bone health specialist.