A bone density test is a quick, painless scan where you lie fully clothed on a padded table while a mechanical arm passes slowly over your body. The whole thing takes 10 to 30 minutes, involves no injections or enclosed spaces, and uses far less radiation than a standard chest X-ray. If you’re picturing something intimidating, the reality is closer to lying still on a firm bed while a camera hovers a few inches above you.
What the Machine Looks Like
The standard bone density test uses a machine called a DXA (sometimes written DEXA), which stands for dual-energy X-ray absorptiometry. The central DXA machine, the type used in hospitals and imaging centers, looks like an open, flat examination table with a large C-shaped arm positioned above it. There’s no tunnel or enclosed tube. The X-ray source sits beneath the table, and a detector rides along the arm overhead. During the scan, the arm glides slowly over the area being measured, usually your lower spine and one hip.
There’s also a smaller, portable version called a peripheral DXA. These compact devices measure bone density at your wrist, heel, or finger. For a wrist scan, you simply sit in a chair and rest your forearm on a small tabletop scanner. These portable units are sometimes available at health fairs or pharmacies, but central DXA of the spine and hip remains the preferred method for diagnosing osteoporosis.
What Happens During the Scan
For a spine measurement, you lie on your back and the technologist places your lower legs on a padded foam block. This elevates your shins, flattens the curve of your lower back, and presses your spine closer to the table for a clearer image. For a hip measurement, your foot is placed in a small brace that gently rotates your leg inward. Neither position is uncomfortable, though it may feel a little awkward.
Once you’re positioned, the arm begins its slow pass. You need to hold very still, and you may be asked to briefly hold your breath, just like with a regular X-ray. You won’t feel anything from the scan itself. There’s no buzzing, no heat, no sensation at all. The technologist operates the machine from a nearby workstation and can see and communicate with you the entire time.
A spine-and-hip scan typically finishes in 10 to 30 minutes depending on the equipment. Some facilities also add a vertebral fracture assessment, which captures a lateral image of your spine to check for compression fractures. That adds only a few extra minutes.
How to Prepare
Preparation is minimal. Stop taking calcium supplements, antacids containing calcium, vitamin D, and multivitamins for 24 hours before your appointment, since calcium in your system can affect the readings. Wear comfortable clothing without metal: no underwire bras, no metal buttons or zippers, no metallic threads or decorative paint. Remove body piercings below the neck if possible. Leave jewelry at home. In most cases you won’t need to change into a gown.
How the Test Measures Your Bones
The DXA machine sends two X-ray beams at different energy levels through your body simultaneously. Bone and soft tissue absorb these two beams differently, which allows the machine to separate bone from everything else in the image. A detector above you captures what comes through, and the software calculates your bone mineral density in grams per square centimeter. The result is a flat, two-dimensional image of the scanned area, not a detailed picture like a CT scan, but enough to precisely measure how dense your bones are.
Radiation Exposure
A DXA scan delivers an extremely small radiation dose. A spine scan exposes you to roughly 0.013 millisieverts (mSv) and a hip scan about 0.009 mSv. For perspective, a standard chest X-ray delivers about 0.02 mSv, and the average person absorbs 2.4 mSv per year just from natural background radiation. A DXA scan of both the spine and hip combined gives you less radiation than a single dental X-ray. An abdominal CT scan, by comparison, delivers about 8 mSv. This makes bone density testing one of the lowest-radiation imaging procedures available.
Understanding Your Results
Your results come 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 defines three ranges:
- Normal: A T-score between +1 and -1
- Low bone mass (osteopenia): A T-score between -1 and -2.5
- Osteoporosis: A T-score of -2.5 or lower
The further your score drops below zero, the lower your bone density and the higher your fracture risk. You may also see a Z-score on your report, which compares you to other people of the same age and sex rather than to a young adult. Z-scores are more commonly used for premenopausal women, men under 50, and children.
Your results are typically available within a few days. If your T-score falls in the osteopenia range, your doctor will likely recommend lifestyle changes and possibly a follow-up scan in one to two years to track any changes. A score in the osteoporosis range usually prompts a conversation about treatment options.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends routine bone density screening for all women aged 65 and older. Postmenopausal women younger than 65 should also consider screening if they have risk factors: low body weight, a parent who fractured a hip, smoking, or heavy alcohol use. These guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or had a fragility fracture.
For men, the evidence is less clear. The Task Force hasn’t issued a firm recommendation for or against routine screening in men, though doctors often order the test for men with specific risk factors like long-term steroid use, low testosterone, or a history of fractures from minor falls.