A DEXA scan (dual-energy X-ray absorptiometry) is a quick, painless imaging test that measures bone density. It’s the gold standard for diagnosing osteoporosis and assessing fracture risk, and it can also provide a detailed breakdown of your body composition, including fat and muscle mass. The scan takes about 10 to 20 minutes, uses very low radiation, and requires almost no preparation.
How the Scan Works
You lie on a padded table while a scanning arm passes over your body. Underneath the table, an X-ray source sends two beams of energy at different levels. One energy level is absorbed primarily by bone mineral, while the other passes more easily through soft tissue. A detector above you captures both signals and uses the difference between them to calculate how dense your bones are.
The result is expressed as bone mass per unit area, measured in grams per square centimeter. The scan typically focuses on two sites: the hip (specifically the femoral neck) and the lower spine, since these are the areas most vulnerable to osteoporotic fractures. Some scans image the whole body, which is especially useful when body composition data is also being collected.
Understanding Your T-Score and Z-Score
Your results come back as two numbers: a T-score and a Z-score. The T-score is the one most people hear about first. It compares your bone density to that of a healthy young adult of the same sex, which represents peak bone mass. The World Health Organization defines three categories based on this score:
- T-score of -1 or higher: Normal, healthy bone density.
- T-score between -1 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.
The Z-score works differently. Instead of comparing you to a young adult, it compares your bone density to other people your own age and sex. This score is particularly useful for younger adults and premenopausal women, where a low T-score might simply reflect that they haven’t yet reached peak bone mass. A Z-score below -2.5 is a red flag that something beyond normal aging may be driving bone loss, such as a hormonal disorder, medication side effect, or nutritional deficiency.
Who Should Get Screened
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 be screened if they have risk factors for osteoporosis, such as a family history of fractures, low body weight, smoking, or long-term use of corticosteroids. These recommendations apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or experienced a fragility fracture.
For men, the evidence is less clear-cut. The Task Force hasn’t issued a firm recommendation for or against routine screening in men, though doctors often order scans for men with specific risk factors like long-term steroid use or low testosterone. Your doctor can help you weigh the benefits based on your individual risk profile.
Body Composition Analysis
Beyond bones, a whole-body DEXA scan can break down your body into three compartments: bone mineral, fat mass, and lean mass. This is far more detailed than stepping on a scale or using a body fat calculator. The scan can show exactly where fat and muscle are distributed across your body, including estimates of visceral fat, the deep abdominal fat surrounding your organs that’s linked to metabolic disease.
Researchers and clinicians use several metrics from these scans. Fat mass index adjusts your total fat for height, similar to how BMI works. Appendicular lean mass index looks specifically at the muscle in your arms and legs relative to your height, which has become a key measurement for diagnosing sarcopenia (age-related muscle loss). Athletes, people tracking body recomposition, and older adults concerned about muscle loss increasingly use DEXA scans for this purpose.
What to Expect on Scan Day
Preparation is minimal. If you take calcium supplements, stop them 24 to 48 hours before the appointment, since undigested calcium in your system can interfere with the readings. Wear loose, comfortable clothing without metal zippers, belts, buttons, or underwire. You’ll likely be asked to remove any jewelry.
During the scan, you lie still on an open table. There’s no enclosed tunnel like an MRI. The scanning arm moves slowly above you, and you won’t feel anything. The whole process typically wraps up in under 20 minutes, and you can go about your day immediately afterward. The radiation exposure is extremely low, a small fraction of what you’d receive from a standard chest X-ray.
How Often to Repeat the Scan
There’s no universal schedule for repeat scans. Current guidelines favor an individualized approach based on your age, baseline bone density, clinical risk factors, and whether you’re on treatment. Medicare covers bone density measurements once every 24 months for eligible patients, and more frequently if medically necessary.
If you’re on medication for osteoporosis, follow-up scans help your doctor assess whether the treatment is working. People at higher risk for rapid bone loss, such as those taking corticosteroids, aromatase inhibitors, or androgen deprivation therapy, or those with inflammatory diseases or early menopause (before age 45), may need scans more frequently. For lower-risk patients on low-dose steroids who aren’t on bone-protective medication, a repeat scan after one to two years is a common recommendation.
Medicare and Insurance Coverage
Medicare Part B covers DEXA scans as a preventive service once every 24 months if you meet at least one qualifying condition. These include being a woman determined to be estrogen-deficient and at risk for osteoporosis, having X-rays suggesting osteoporosis or vertebral fractures, taking or planning to take steroid-type drugs, having been diagnosed with primary hyperparathyroidism, or being monitored during osteoporosis treatment.
Most private insurance plans follow similar criteria, though specifics vary. Body composition scans done purely for fitness tracking, rather than for a medical indication, are less likely to be covered and may need to be paid out of pocket.