A bone density test is a quick, painless scan that measures how strong and thick your bones are. It uses a special type of X-ray to detect bone loss before you break a bone, and it’s the primary tool for diagnosing osteoporosis. The most common version, called a DEXA scan (dual-energy X-ray absorptiometry), takes about 10 to 20 minutes and exposes you to less radiation than a standard chest X-ray.
How the Scan Works
A DEXA scan passes two X-ray beams at different energy levels through your body. Because bone, muscle, and fat each absorb these beams differently, the machine can calculate exactly how dense your bones are. The areas most commonly scanned are the lower spine, the narrow neck of the thighbone near the hip joint, and sometimes the forearm. These are the bones most likely to fracture from osteoporosis.
During the scan, you lie on a flat, open table while a scanning arm passes over your body. A technologist will position your legs on a padded box and may place your foot in a brace to rotate your hip slightly inward. There’s no tunnel or enclosed space, and you won’t feel anything from the X-ray beams. The whole process is over in minutes.
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 be screened if they have risk factors that raise their chance of fracture, such as low body weight, a family history of osteoporosis, smoking, or long-term use of corticosteroid medications.
For men, the evidence is less clear-cut. There is no formal screening recommendation for men at average risk, though doctors often order the test for men with specific concerns: a history of fractures, conditions like rheumatoid arthritis, or medications known to weaken bones. These guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or had a fragility fracture.
How to Prepare
Preparation is minimal. Wear loose, comfortable clothing without metal zippers, buttons, or buckles, since metal interferes with the scan. You’ll need to remove any jewelry in the scanning area. If you take calcium supplements or other over-the-counter vitamins, stop taking them 24 hours before your appointment. There’s no fasting required and no injections involved.
Understanding Your T-Score
Your results come back as a number called a T-score, which compares your bone density to that of a healthy 30-year-old adult at peak bone mass. The scale works like this:
- T-score of -1 or higher: Normal, healthy bone density.
- T-score between -1 and -2.5: Osteopenia, meaning your bones are thinner than normal but not yet in the osteoporosis range.
- T-score of -2.5 or lower: Osteoporosis, indicating significant bone loss and a higher risk of fracture.
Every full point drop in your T-score represents a meaningful decrease in bone strength. Someone with a score of -2.0 has noticeably weaker bones than someone at -1.0, even though both fall in the osteopenia category.
You may also see a Z-score on your report. While a T-score compares you to a young adult, a Z-score compares you to other people of your same age, sex, and body size. Z-scores are most useful for younger adults or when a doctor suspects bone loss is being caused by an underlying medical condition rather than normal aging.
What Your Doctor Does With the Results
A T-score alone doesn’t always tell the full story. Many doctors plug your results into a tool called FRAX, which calculates your 10-year probability of breaking a hip or having another major fracture. FRAX combines your T-score with factors like your age, sex, BMI, and seven clinical risk factors (including smoking history, alcohol use, and whether a parent broke a hip). Of all the inputs, age and T-score contribute the most to the final risk estimate.
Treatment is generally recommended when FRAX shows a 10-year hip fracture risk of 3% or higher, or a 10-year risk of any major osteoporotic fracture of 20% or higher. This means two people with the same T-score could get different treatment recommendations based on their other risk factors.
Radiation Exposure
A DEXA scan delivers an extremely small dose of radiation. For a standard spine-plus-hip scan, the effective dose ranges from less than 1 microsievert to about 15 microsieverts, depending on the machine type. To put that in perspective, you absorb roughly 10 microsieverts of natural background radiation every day just from your environment. A chest X-ray delivers 20 to 50 microsieverts, so most DEXA scans expose you to less radiation than a single chest X-ray. This makes it one of the lowest-dose imaging tests available.
What Can Affect Accuracy
DEXA scans are highly reliable, but a few things can throw off the numbers. Advanced arthritis in the spine is the most common culprit. Bone spurs and narrowed discs can make vertebrae appear denser than they actually are, potentially masking real bone loss. This tends to affect spine measurements specifically; hip readings are less likely to be skewed by arthritis. If you have significant spinal arthritis, your doctor may rely more on your hip score or order a forearm scan instead.
Previous fractures in the scanning area, metal implants, or recent use of contrast dye from another imaging test can also interfere. Let your technologist know about any of these before the scan begins.
How Often to Repeat the Test
If your first scan shows normal bone density, you likely won’t need another one for several years. The exact interval depends on your initial results and risk profile. Someone with a T-score close to -1 may benefit from rescanning sooner than someone whose score is well above that threshold, since they’re closer to the osteopenia range. If you’re being treated for osteoporosis, follow-up scans are typically done every one to two years to track whether treatment is working. When comparing scans over time, it’s best to use the same machine at the same facility, since slight differences between scanners can make it hard to detect real changes in bone density.