A bone density test measures how much mineral, primarily calcium, is packed into a segment of your bone. The result tells you whether your bones are strong, thinning, or fragile enough to qualify as osteoporosis. Most people get a single number called a T-score, which compares your bone density to that of a healthy 30-year-old at peak bone strength.
What Your T-Score Means
The core output of a bone density test is your T-score, and the scale is straightforward. A T-score of -1 or higher means your bone density is healthy. A score between -1 and -2.5 means you have osteopenia, a milder form of bone loss that signals your bones are thinning but haven’t crossed into dangerous territory. A T-score of -2.5 or lower indicates osteoporosis, where bones have lost enough density to fracture more easily.
Every full point on the T-score scale represents a meaningful change in fracture risk. Someone with a score of -2 has noticeably weaker bones than someone at -1, even though both fall in the osteopenia range. Your report will usually show separate T-scores for each site that was scanned, so you might have a -0.8 at the spine and a -1.4 at the hip. The lowest score across all sites is typically the one used for diagnosis.
Where on Your Body the Test Measures
The standard scan targets your lower spine and hip, because these are the areas most vulnerable to osteoporotic fractures and most representative of your overall skeletal health. These central scans use a full-sized machine you lie down on. The whole process takes about 10 to 20 minutes, and you stay fully clothed.
Smaller portable devices can scan your wrist, heel, or finger. These peripheral tests are quicker and sometimes used as initial screenings at health fairs or pharmacies, but they aren’t considered precise enough to diagnose osteoporosis or track changes over time. If a peripheral test flags a concern, you’ll likely be sent for a full spine and hip scan.
How the Scan Works
The technology behind the test is called dual-energy X-ray absorptiometry, commonly shortened to DXA (sometimes written DEXA). It sends two X-ray beams at different energy levels through your body. Bone and soft tissue absorb these beams differently, which lets the machine isolate just the mineral content of your bones and calculate density.
Radiation exposure is minimal. A spine-plus-hip scan delivers roughly 1 to 15 microsieverts, which is comparable to or less than the amount of background radiation you absorb from the natural environment in a single day. For comparison, a standard chest X-ray delivers several times more radiation than a DXA scan.
Beyond the T-Score: Fracture Risk
A bone density test shows how dense your bones are right now, but density alone doesn’t capture your full fracture risk. That’s why doctors often plug your T-score into a tool called FRAX, which estimates your probability of breaking a bone in the next 10 years. FRAX combines your hip bone density with several other factors: your age, sex, body weight relative to height, whether you’ve broken a bone before, whether a parent broke a hip, steroid use, smoking status, alcohol intake (three or more drinks per day raises risk), and whether you have rheumatoid arthritis or other conditions that contribute to bone loss.
Two people with identical T-scores can have very different fracture risks depending on these additional factors. A 55-year-old with a T-score of -1.8 and no other risk factors is in a different situation than a 70-year-old with the same score who smokes and has a family history of hip fractures. The FRAX calculation helps determine whether treatment is worth starting, especially for people in the osteopenia range where the decision isn’t as clear-cut.
What the Test Cannot Show
DXA scans have real limitations worth understanding. The test measures mineral content across a flat, two-dimensional image of a three-dimensional bone. It can’t distinguish between the dense outer shell of bone and the spongy interior lattice, and these two types of bone lose density at different rates and respond differently to treatment. It also can’t evaluate bone quality: the microscopic architecture, the accumulation of tiny cracks, or how well collagen fibers hold the mineral matrix together. Two bones with the same density reading can differ significantly in actual strength.
DXA also tends to underestimate density in smaller-framed people and overestimate it in larger-framed people, simply because it’s projecting a 3D structure onto a 2D measurement. Arthritis, prior fractures, or calcium deposits in the spine can artificially inflate your score in those areas, making bones look denser than they truly are.
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 also be screened if they have elevated risk factors, such as low body weight, smoking, family history of fractures, or long-term steroid use. For men, there’s no universal screening recommendation because the evidence on benefits and harms isn’t strong enough yet, though individual doctors may order the test based on a man’s specific risk profile.
These guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or experienced a fragility fracture. People with conditions known to cause bone loss, like certain cancers, thyroid disorders, or chronic use of medications such as glucocorticoids, typically follow a different testing schedule determined by their treatment plan.
Tracking Bone Changes Over Time
If you’re diagnosed with osteopenia or osteoporosis, repeat scans let your doctor see whether your bones are holding steady, continuing to thin, or responding to treatment. The testing interval depends on your situation. For people on osteoporosis medication, repeat scans are commonly done every one to two years to check whether treatment is working.
If you stop medication, timing matters too. Some treatments have a longer residual effect on bone than others. After pausing certain shorter-acting medications, bone density may need to be rechecked within a year, while longer-acting treatments may hold their effect for two to three years before a repeat scan is necessary. For people with normal or mildly low results and no new risk factors, retesting every few years is usually sufficient.
Preparing for the Test
Preparation is simple. You should stop taking calcium supplements for at least 24 hours before the scan, since undigested calcium in your system can interfere with accuracy. Wear comfortable clothing without metal zippers, buttons, or belt buckles, as you’ll stay dressed during the scan. If you’ve had a barium study or received contrast dye for a CT scan or MRI recently, let the scheduling office know, as residual contrast material can affect the reading.
During the scan itself, you lie on a padded table while a scanning arm passes over you. There’s no injection, no enclosed space, and no discomfort. Results are typically available within a few days, sometimes sooner.