A bone density test is a quick, painless scan that uses low-dose X-rays to measure how much mineral is packed into your bones. The most common version, called a DXA scan (dual-energy X-ray absorptiometry), takes about 10 to 30 minutes, requires no injections or special preparation, and delivers less radiation than a standard chest X-ray. Here’s what to expect before, during, and after.
How the Scan Works
The DXA machine sends two thin X-ray beams through the area being scanned. Each beam has a different energy level: one is absorbed mainly by soft tissue, and the other mainly by bone. The machine subtracts the soft tissue reading from the total, and what remains is your bone mineral density. This two-beam approach is what makes DXA more precise than a regular X-ray, which can show a fracture but can’t quantify how dense your bones actually are.
Central DXA vs. Peripheral Tests
A central DXA scan measures the hip and lower spine, the two areas most vulnerable to osteoporotic fractures. This is the gold standard test, performed on a large flat table in a radiology office or hospital. Peripheral devices (sometimes called pDXA) measure smaller bones like the wrist, heel, or finger and are occasionally offered at health fairs or pharmacies. Peripheral tests can flag potential problems, but they aren’t precise enough to diagnose osteoporosis or monitor treatment. If a peripheral test suggests low bone density, you’ll still need a central DXA for confirmation.
What Happens During the Test
You lie flat on your back on a padded table. For the spine measurement, a foam block is placed under your lower legs to flatten your pelvis and lower back against the table. For the hip measurement, your foot is placed in a brace that rotates your hip slightly inward. A scanning arm passes slowly above you, and you’ll need to stay still for each scan, which typically lasts a few minutes per site. There’s no tunnel or enclosed space, and most people describe the experience as uneventful.
The entire appointment, including positioning and scanning both hip and spine, usually takes 10 to 30 minutes. You won’t feel the X-ray beam, and there are no side effects afterward. You can eat normally, drive yourself home, and go about your day.
How to Prepare
Preparation is minimal. Stop taking calcium supplements or multivitamins containing calcium 24 hours before your test, since undigested calcium in your system can interfere with accuracy. Wear comfortable, loose-fitting clothes without metal zippers, buttons, or buckles. You’ll need to remove any jewelry or clothing with metal in the scan area.
If you’ve had any of the following within the past seven days, let your doctor’s office know before scheduling:
- A barium study (such as a barium swallow or barium enema)
- A CT or MRI scan with contrast dye
- A nuclear medicine study
Residual contrast material or radioactive tracers can throw off the DXA reading, so you’ll need to wait at least seven days after any of these procedures.
How Much Radiation You’re Exposed To
Very little. A spine-plus-hip DXA scan delivers roughly 1 to 15 microsieverts of radiation, depending on the type of machine. To put that in perspective, you absorb about 10 microsieverts from natural background radiation in a single day just going about your life. A chest X-ray delivers 20 to 50 microsieverts. Most modern DXA systems expose you to less radiation than that one chest X-ray, and pencil-beam scanners (the most common type) often deliver less than 1 microsievert per scan.
Understanding Your Results
Your results come back as a T-score, which compares your bone density to the peak bone mass of a healthy 30-year-old of the same sex. The scale works 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
You may also receive a Z-score, which compares you to other people of your same age, sex, and body size. Z-scores are more useful for younger adults and children, where a T-score comparison to peak bone mass wouldn’t be as meaningful.
Your T-score alone doesn’t determine your full fracture risk. Doctors often combine it with a fracture risk calculator that factors in your age, body weight, smoking status, alcohol intake, whether a parent broke a hip, whether you’ve had a prior fracture, and whether you take certain medications like oral steroids. These clinical risk factors can raise your fracture probability independently of bone density. Someone with a modest T-score but multiple risk factors may have a higher chance of fracture than someone with a lower T-score and no other risks.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends routine bone density screening for all women 65 and older. Postmenopausal women younger than 65 should be screened if they have additional risk factors, including low body weight, a parent who broke a hip, cigarette smoking, or heavy alcohol use. For women in that younger group, doctors typically use a clinical risk tool to decide whether scanning is warranted.
For men, there’s no blanket screening recommendation. The evidence hasn’t been strong enough to establish a clear benefit-to-harm ratio for routine male screening. That said, men with specific risk factors (long-term steroid use, heavy drinking, a history of fractures, or conditions that affect bone health) are often tested on an individual basis.
How Often You’ll Need Repeat Scans
If your results are normal and you have no major risk factors, you likely won’t need another scan for several years. Medicare covers bone density testing once every 24 months, or more frequently if medically necessary. If you’ve been diagnosed with osteoporosis and started treatment, repeat scans are typically done at that two-year interval to check whether your bone density is stabilizing or improving. Your doctor may recommend more frequent monitoring if your initial results were borderline or if your risk profile changes.