A bone density test reveals how much mineral is packed into your bones, expressed as a score that tells you whether your bones are normal, thinning, or weak enough to qualify as osteoporosis. But the test can show more than just a single number. Depending on how it’s performed, it can also detect spinal fractures you didn’t know you had, estimate your risk of breaking a bone in the next decade, and even assess the internal structure of your bone tissue.
The Core Measurement: Bone Mineral Density
The standard bone density test is called a DXA (sometimes written DEXA), which stands for dual-energy X-ray absorptiometry. It measures how dense your bones are at specific sites, most commonly the hip and spine, since those are the bones most likely to fracture from osteoporosis. Some tests also scan the forearm. The result is a measurement of mineral content per unit of bone area, reported in grams per square centimeter.
That raw number gets translated into two scores that appear on your results:
- T-score: This compares your bone density to a healthy young adult at peak bone mass. A T-score of 0 means your density matches that reference. A negative number means your bones are less dense. A T-score between -1 and -2.5 indicates low bone mass, called osteopenia. A T-score of -2.5 or lower means osteoporosis.
- Z-score: This compares your bone density to other people your same age, sex, and body size. It’s especially useful for younger adults and can flag whether something beyond normal aging is causing bone loss.
Your report typically shows separate scores for each site scanned. It’s common to have different readings at the hip and spine, since bone loss doesn’t happen evenly throughout the skeleton. The lowest T-score from any site is generally the one used for diagnosis.
Fracture Risk Estimates
If your results show low bone mass (osteopenia), your report may include a FRAX score. This is a 10-year fracture risk estimate developed by the World Health Organization that combines your bone density with other personal risk factors: your age, weight, smoking status, alcohol use, whether a parent broke a hip, and whether you take certain medications.
The FRAX score gives you two numbers: your chance of breaking a hip over the next 10 years, and your combined chance of breaking a hip, spine, forearm, or shoulder. These percentages help you and your provider decide whether preventive treatment makes sense, particularly when your T-score falls in that gray zone between normal and osteoporosis.
Hidden Spinal Fractures
One of the more surprising things a bone density test can reveal is a spinal compression fracture you never knew about. Many DXA machines can perform what’s called a Vertebral Fracture Assessment (VFA), which takes a lateral image of your spine during the same appointment. This picks up vertebrae that have lost height due to compression, even if you never felt a sudden “break.”
These fractures are classified by shape and severity. A wedge fracture shows height loss at the front of the vertebra. A biconcave fracture compresses the middle while the front and back stay intact. A crush fracture compresses the entire vertebral body. Severity is graded by how much height the vertebra has lost: mild fractures involve 20 to 25 percent loss, moderate fractures 25 to 40 percent, and severe fractures more than 40 percent.
Finding even one of these fractures changes the clinical picture significantly, because a previous vertebral fracture is one of the strongest predictors of future fractures. A person with osteopenia and a hidden spinal fracture faces a very different level of risk than someone with the same T-score and no fractures. That’s why VFA captures two independent risk factors in a single visit: bone density and the presence of existing fractures.
Bone Quality, Not Just Density
A newer feature available on some DXA machines is the Trabecular Bone Score, or TBS. Standard bone density tells you how much mineral is in your bones, but TBS goes a step further by analyzing the texture and microarchitecture of your spine’s spongy inner bone tissue. Think of it as the difference between measuring how much wood is in a bridge versus checking whether the wood is solid or riddled with gaps.
TBS uses software installed on the DXA machine to analyze the same spine image already captured during a standard scan. Research from the Mayo Clinic has shown that TBS independently predicts fracture risk and is particularly valuable for people whose T-scores look borderline but whose bone quality may be worse than the number suggests. This is especially relevant for people with conditions like diabetes or those taking long-term steroids, where bones can be fragile even when density readings appear reasonable.
Things That Can Throw Off the Results
A DXA scan measures everything in the path of its X-ray beam, not just bone. That means calcified tissue outside the skeleton can artificially inflate your bone density reading, making your bones appear stronger than they actually are.
The most common culprits are calcification of the aorta (the large artery that runs alongside the spine) and calcified intervertebral discs. In cadaver studies, removing a calcified aorta decreased measured spine density by an average of nearly 5 percent, and removing calcified discs dropped it by an additional 12 percent. Degenerative arthritis in the spine, which is extremely common in older adults, can also add extra density to the reading. Surgical hardware, prior fractures, and even severe scoliosis can distort results at specific vertebrae.
Technicians and radiologists account for these issues by excluding affected vertebrae from the calculation when possible. If your lower spine is heavily affected by arthritis or prior surgery, a hip measurement may give a more accurate picture. Your report should note any vertebral levels that were excluded and why.
Who Gets Tested and When
The U.S. Preventive Services Task Force recommends routine bone density screening for all women 65 and older. Postmenopausal women younger than 65 should be screened if they have risk factors like low body weight, a parent who broke a hip, smoking, or heavy alcohol use. For men, there isn’t enough evidence yet for the task force to make a universal recommendation, though individual providers may order testing based on personal risk factors.
These guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or had a fragility fracture. People with conditions that cause secondary bone loss, like certain cancers, thyroid disorders, or long-term steroid use, fall outside these general recommendations and typically follow a different screening schedule set by their specialist.
Follow-up scans are usually spaced at least two years apart, since bone density changes slowly and the test needs enough time between scans to detect a meaningful difference beyond its own margin of measurement error.