A DXA scan (dual-energy X-ray absorptiometry) is a quick, painless imaging test that measures bone density and body composition. It uses two low-dose X-ray beams at different energy levels to distinguish bone from soft tissue, producing a detailed picture of how strong your bones are. The scan takes 10 to 30 minutes, delivers less radiation than a standard chest X-ray, and is the primary tool used to diagnose osteoporosis and assess fracture risk.
How the Scan Works
The technology behind a DXA scan is straightforward. The machine sends two X-ray beams through your body, each at a different energy level. Bone and soft tissue absorb these beams differently, and the machine’s software uses that difference to calculate exactly how dense your bones are. Some machines alternate between high and low voltage as the scanner moves across your body, while others emit a constant beam and use a filter to separate the two energy levels. Either way, the result is a precise measurement of bone mineral density at the sites most vulnerable to fracture.
What Happens During the Test
You’ll lie on a padded table while a scanning arm passes over your body. The most common areas scanned are the lower spine and hips, since these are the sites where osteoporotic fractures cause the most damage. In some cases, particularly for children or when assessing body composition, a whole-body scan is performed instead. Peripheral devices that measure the wrist, heel, or finger also exist, but they’re generally used only for initial screening rather than diagnosis.
The scan itself takes 10 to 30 minutes depending on the equipment and the areas being examined. You won’t feel anything. There are no injections, no enclosed spaces, and no loud noises.
How to Prepare
Preparation is minimal, but a few details matter. Stop taking calcium supplements, antacids containing calcium, vitamin D, and multivitamins for 24 hours before your appointment. You can still eat calcium-containing foods normally. If you’ve recently had an imaging test that used contrast dye (such as a CT scan with contrast, an MRI with gadolinium, or a nuclear medicine scan), you’ll need to wait at least three full days before your DXA. For tests involving oral barium contrast, the wait is seven days.
On the day of the scan, avoid clothing with metal buttons, zippers, or metallic threads, and skip underwire bras. Remove any body piercings below the neck if possible, since metal interferes with the X-ray readings.
Radiation Exposure
DXA scans use remarkably little radiation. A spine-plus-hip scan delivers an effective dose ranging from less than 1 microsievert to about 15 microsieverts. For comparison, a single chest X-ray exposes you to 20 to 50 microsieverts. That means most DXA scans give you less radiation than the chest X-ray your doctor might order for a cough.
Understanding Your T-Score
Your results come back as a T-score, which compares your bone density to the peak bone density of a healthy young adult. The World Health Organization defines three categories:
- 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 falls below zero, the lower your bone density and the higher your fracture risk. A T-score of -3.0, for example, represents significantly more bone loss than -1.5.
If you’re younger or premenopausal, your results may also include a Z-score, which compares your bone density to others of the same age and sex rather than to peak bone density. This helps identify whether bone loss is unusual for your age group and might point to an underlying cause.
How Results Predict Fracture Risk
Your T-score often gets plugged into a calculator called FRAX, which estimates your 10-year probability of breaking a hip or sustaining another major fracture (spine, forearm, or upper arm). FRAX combines your hip bone density measurement with clinical risk factors like age, sex, body weight, smoking status, alcohol use, parental history of hip fracture, and certain medications such as corticosteroids. The output is a percentage: your personal likelihood of fracture over the next decade. That number helps guide decisions about whether treatment is warranted or whether monitoring alone is enough.
Hip bone density is particularly valuable in this calculation. Research consistently shows that hip measurements are the strongest predictor of hip fractures specifically, though spine and hip results together give the most complete picture of your skeletal health.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends DXA screening for all women aged 65 and older. Postmenopausal women younger than 65 should also be screened if they have risk factors: low body weight, a parent who fractured a hip, smoking, excessive alcohol use, or use of medications like corticosteroids or insulin. For men, there isn’t enough evidence yet for a universal screening recommendation, though doctors often order the test for men with clear risk factors or a history of fractures.
Central vs. Peripheral Devices
The large table-based machines found in hospitals and imaging centers are called central DXA scanners, and they measure the spine and hip directly. Smaller portable devices, known as peripheral scanners, measure sites like the wrist, heel, or finger. These peripheral devices can flag low bone mass, but they have limitations. Measurements at the wrist don’t correlate well with what’s happening at the hip or spine, and there’s no consensus on how to interpret peripheral results the same way clinicians interpret central DXA scores. If a peripheral scan suggests a problem, you’ll typically be sent for a central DXA to confirm the diagnosis.
Body Composition Analysis
DXA isn’t only for bones. A whole-body scan can break down your body into fat mass, lean mass (muscle, organs, connective tissue), and bone mineral content with a level of precision that standard scales and calipers can’t match. This makes it increasingly popular among athletes, people managing weight loss, and clinicians evaluating conditions like sarcopenia (age-related muscle loss).
A body composition report typically includes your total body fat percentage, fat mass index (your total fat relative to your height), fat-free mass index (everything that isn’t fat relative to your height), and an estimate of skeletal muscle mass. Many reports also calculate your android-to-gynoid fat ratio, which describes how your fat is distributed between your midsection and hips. Values above 0.8 for women or 1.0 for men suggest a pattern of central fat storage associated with higher cardiovascular and metabolic risk.
Some scans also estimate resting metabolic rate based on the proportions of different tissues in your body, giving you a baseline for how many calories your body burns at rest. This is a modeled estimate rather than a direct measurement, but it’s useful as a reference point for nutrition planning.