A DXA scan (dual-energy X-ray absorptiometry) is a low-radiation imaging test that measures bone density to determine your risk of osteoporosis and fractures. It’s the standard tool doctors use to diagnose bone loss, and it can also measure body composition, including fat and muscle distribution. The scan takes about 15 to 20 minutes, uses less radiation than a standard chest X-ray, and requires no injections or special preparation beyond skipping calcium supplements the day before.
How the Scan Works
A DXA machine uses a C-arm that sends two X-ray beams at different energy levels through your body while you lie on a flat table. One energy level is absorbed primarily by soft tissue, the other by bone. A detector above you captures what passes through, and the machine combines these readings into a flat image that calculates your bone mass per unit of area, measured in grams per square centimeter.
The dual-energy approach is what makes DXA more precise than a regular X-ray for bone assessment. By comparing how each beam is absorbed, the machine can isolate bone from the surrounding muscle and fat and produce a reliable density measurement. The technology has been in clinical use since the FDA first approved it in 1988.
What Happens During the Scan
You’ll lie flat on an open table, not inside an enclosed tube, so it’s nothing like an MRI. A technologist will position your legs on a padded box and may place your foot in a brace to rotate your hip slightly inward. The scanning arm passes slowly over your lower spine and one or both hips. You just need to stay still and breathe normally.
The whole appointment typically lasts 15 to 20 minutes. There’s no pain, no noise, and no contrast dye. To prepare, stop taking calcium supplements at least 24 hours before the exam and wear clothes without metal zippers, buttons, or snaps. Shorts and a T-shirt work well. You’ll also need to remove any belts or jewelry near the scan area.
Understanding Your T-Score
Your results come back as a T-score, which compares your bone density to that of a healthy 30-year-old adult at peak bone mass. The World Health Organization classifies the numbers like this:
- T-score of -1.0 or higher: Normal, healthy bone density.
- T-score between -1.0 and -2.5: Osteopenia, meaning bone density is lower than normal but not yet in the osteoporosis range.
- T-score of -2.5 or lower: Osteoporosis, indicating significantly reduced bone density and higher fracture risk.
A T-score is straightforward to read because each full point represents one standard deviation from the reference value. So a score of -2.0 means your bone density is two standard deviations below the young-adult average. The lower the number, the greater your fracture risk. Your doctor may also report a Z-score, which compares you to others your own age and sex rather than to a 30-year-old. Z-scores are more useful for younger adults and children, where a low reading relative to peers may signal an underlying condition affecting bone health.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends routine osteoporosis screening for all women aged 65 and older. Postmenopausal women younger than 65 should also be screened if they have increased fracture risk based on factors like low body weight, a parent who fractured a hip, smoking, or heavy alcohol use. For men, there isn’t yet enough evidence for a blanket screening recommendation, but doctors often order the scan for men with specific risk factors like long-term steroid use or a history of fractures.
Medicare covers bone density testing once every 24 months for people who meet certain criteria: women determined to be estrogen-deficient and at risk, anyone whose X-rays suggest bone loss or vertebral fractures, people taking or starting steroid medications like prednisone, those diagnosed with primary hyperparathyroidism, and patients being monitored to see if osteoporosis treatment is working. More frequent testing is covered when deemed medically necessary.
How Often to Repeat the Scan
Follow-up timing depends on your individual situation. The International Society for Clinical Densitometry updated its guidance in 2023 to emphasize that repeat scans should be scheduled based on your age, sex, fracture risk, and whether you’re on treatment, rather than following a rigid timeline for everyone. A follow-up DXA should have a clear purpose: either checking whether a medication is preserving or rebuilding bone, or reassessing risk after a new fracture or a change in health status.
In practice, many people with normal or mildly low bone density get rescanned every two to five years. Someone starting osteoporosis medication might have a follow-up in one to two years to confirm the treatment is working. If your initial scan is normal and you have no major risk factors, your doctor may wait several years before repeating it.
Radiation Exposure
DXA uses remarkably little radiation. A spine-plus-hip scan delivers roughly 1 to 15 microsieverts depending on the machine type. For context, you absorb about 10 microsieverts from natural background radiation in a single day, and a chest X-ray delivers 20 to 50 microsieverts. Older pencil-beam scanners come in under 1 microsievert, while newer fan-beam and cone-beam systems run slightly higher, up to about 18 microsieverts. Even at the upper end, the dose is comparable to or less than a chest X-ray.
What Can Throw Off Your Results
DXA is highly reliable, but certain conditions can make bone density appear falsely high. Osteoarthritis in the spine is the most common culprit. Bone spurs and joint thickening from arthritis add extra dense material in exactly the area being measured, inflating your score. A fractured vertebra also reads as denser than it actually is because the compressed bone packs more tightly together, raising the measurement by an average of 0.070 g/cm² at that spot.
Surgical hardware creates obvious problems. Metal screws, rods, fusion material, or spinal clips in the scan area make that site unusable for diagnosis. If you have a hip replacement on one side, the technologist scans the opposite hip instead. Even after hardware is removed, sclerotic changes in the bone can linger and affect readings. Conditions like ankylosing spondylitis, where the spine gradually fuses, also artificially elevate spine measurements.
To account for these issues, guidelines say that any vertebra showing major structural changes, or differing by more than one T-score from its neighbor, should be excluded from the calculation. This is why your results may report on only two or three lumbar vertebrae rather than all four.
Body Composition Analysis
Beyond bone density, a whole-body DXA scan can break down your body into three compartments: bone, lean soft tissue, and fat. The machine subdivides you into regions (arms, legs, trunk, and head) and reports the composition of each. This gives you a detailed picture that a bathroom scale or even calipers can’t provide.
Several useful metrics come from this data. Fat mass index normalizes your total fat by height squared, similar to how BMI works but using actual measured fat rather than total weight. Appendicular lean mass index takes the lean tissue in your arms and legs, divides by height squared, and produces a number used to screen for sarcopenia, the age-related loss of muscle mass. Many DXA systems now also estimate visceral fat, the metabolically active fat packed around your organs, which is a stronger predictor of cardiovascular and metabolic risk than overall body fat percentage.
Body composition scans are increasingly used by athletes, older adults tracking muscle loss, and people managing conditions like obesity or diabetes where the distribution of fat and muscle matters more than weight alone.