A DEXA scan is a low-dose X-ray that measures bone density and body composition. Short for dual-energy X-ray absorptiometry, it’s the standard test doctors use to diagnose osteoporosis and assess fracture risk. The scan takes about 30 minutes, exposes you to less radiation than a chest X-ray, and requires almost no preparation.
How the Scan Works
DEXA uses two X-ray beams at different energy levels to distinguish bone from soft tissue. Bone absorbs the higher-energy beam and reduces its signal, while soft tissue lets most of it pass through. By comparing how much each beam is absorbed at every point in the scan, the machine creates a detailed map of your bone mineral content. This two-beam approach is what makes DEXA more precise than a standard X-ray for measuring density.
The same principle allows DEXA to separate fat from lean tissue in soft-tissue areas, which is why the technology pulls double duty as both a bone density test and a body composition analysis.
What Happens During the Test
You lie on a padded table while a scanning arm passes over your body. Technicians typically scan your hips and lumbar spine, since these are the most common fracture sites in osteoporosis. They may also scan your forearms or other bones depending on your situation. The whole process lasts about 30 minutes, sometimes less. You won’t feel anything from the X-rays, and there’s no enclosed space like an MRI.
Preparation is minimal. Stop taking calcium supplements at least 24 hours before the test. Wear loose, comfortable clothing without zippers, belts, or metal buttons. Leave jewelry at home and empty metal objects from your pockets.
Radiation Exposure
A spine and hip DEXA scan delivers between 1 and 20 microsieverts of radiation, depending on the machine and scan mode. For context, you absorb about 7 to 8 microsieverts per day just from natural background radiation. A chest X-ray delivers roughly 20 microsieverts, and a cross-country flight exposes you to about 40. DEXA is one of the lowest-radiation imaging tests available.
Understanding Your T-Score
The main number on a bone density report is the T-score, which compares your bone density to that of a healthy 30-year-old adult. The World Health Organization defines three categories:
- T-score of -1.0 or higher: Normal, healthy bone density
- T-score between -1.0 and -2.5: Osteopenia, a milder form of bone loss that raises fracture risk but isn’t yet osteoporosis
- T-score of -2.5 or lower: Osteoporosis
Your report may also include a Z-score, which compares your density to other people of the same age, sex, and ethnicity. Z-scores are more useful for younger adults and can flag whether bone loss is happening faster than expected for your age group, which sometimes points to an underlying medical condition.
Body Composition Analysis
Beyond bone, a DEXA scan can break your body into three components: fat, muscle, and bone. This type of scan is increasingly popular in sports medicine and fitness settings because it provides detail that a scale or even calipers can’t match.
A body composition report typically includes your total body fat percentage, lean mass in each limb, and fat mass index (total fat relative to your height). One particularly useful metric is visceral adipose tissue, the fat stored deep in your abdomen around your organs. Unlike the fat you can pinch under your skin, visceral fat is hormonally active and strongly linked to cardiovascular and metabolic disease risk. DEXA is one of the few accessible tools that can quantify it directly.
The scan also calculates your appendicular lean mass relative to your height or BMI. This ratio helps identify sarcopenia, a condition of dangerously low muscle mass. Research cut points for sarcopenia risk are generally around 7.0 for men and 5.5 for women when lean mass is measured against height, or 0.79 for men and 0.51 for women when measured against BMI.
Who Should Get a DEXA Scan
The U.S. Preventive Services Task Force recommends routine bone density screening for all women 65 and older. Postmenopausal women younger than 65 should also be screened if they have elevated fracture risk. Key risk factors include low body weight, a parent who fractured a hip, smoking, and excess alcohol consumption. The task force suggests a two-step approach: first identify whether risk factors are present, then screen if they are.
For men, there’s no blanket screening recommendation. The task force says there isn’t enough evidence yet to weigh the benefits and harms of routine screening in men, though doctors may still order the test for men with specific risk factors.
Certain medical conditions also warrant a DEXA scan regardless of age or sex. These include hyperparathyroidism, Cushing’s syndrome, and conditions affecting the pituitary gland. Long-term use of corticosteroids, which accelerate bone loss, is another common reason doctors order the test.
Accuracy and Limitations
DEXA is considered the gold standard for bone density measurement, but the results aren’t immune to error. A study at a university hospital in Italy reviewed 485 DEXA exams and found that 93% contained at least one error. The vast majority (79%) were data analysis errors, meaning mistakes in how the software or technician processed the scan after it was taken. Another 12% were positioning errors, 7% involved artifacts that distorted the reading, and 2% were simple demographic mistakes like an incorrect birth date, which throws off your T-score and Z-score calculations.
Spinal arthritis, compression fractures, and surgical hardware can all inflate your spine’s apparent density, making bones look healthier than they are. This is one reason technicians scan multiple sites. If your spine results seem inconsistent with the rest of your body, your doctor may rely more heavily on hip measurements or order imaging of the forearm instead.
When tracking bone density over time, small changes between scans can fall within the machine’s margin of error. Meaningful comparison requires using the same machine (or at least the same manufacturer) each time, and changes of less than 3 to 5 percent may not reflect a true shift in bone health.