What Is a DEXA Scan? Bone Density Testing Explained

A DEXA scan (also written DXA, for dual-energy X-ray absorptiometry) is a low-radiation imaging test that measures how dense your bones are. Introduced in the 1980s, it remains the standard tool for diagnosing osteoporosis, predicting fracture risk, and tracking whether bone-protecting treatments are working.¹ The scan itself is quick, painless, and involves far less radiation than a standard chest X-ray, but the numbers it produces can be confusing if nobody walks you through them.

What Happens During the Scan

You lie on a padded table, fully clothed (though you may be asked to change into a gown if your clothes have metal zippers or buttons). An arm mounted above the table passes slowly over the area being measured, usually the lower spine and one or both hips. Some clinics also scan the forearm. The machine sends two X-ray beams at different energy levels through your body. Bone absorbs more of these beams than soft tissue does, and by comparing how much of each beam makes it through, the scanner calculates your bone mineral density at each site.

The whole process typically takes ten to twenty minutes. There is no enclosed tube, no injection, no contrast dye, and no sensation at all from the beams. Because the scan relies on precise positioning, the technologist will position your legs or arms in a specific way and ask you to hold still. If you are having a follow-up scan to monitor changes over time, getting scanned on the same machine each visit matters, since different manufacturers’ devices can produce slightly different readings.

A Note on Preparation

DEXA scans require little preparation, but one detail trips people up: calcium supplements taken shortly before a spine scan can throw off the measurement. A study found that about a third of subjects showed bone density changes outside the expected range within 30 minutes of taking a calcium tablet, whether or not the tablet was visible on the scan image.² The practical takeaway is simple: skip your calcium supplement on the morning of the scan or at least avoid taking it within a few hours of your appointment. No fasting is required otherwise.

How to Read Your Results

Your DEXA report centers on two numbers: a T-score and a Z-score. Both compare your bone density to a reference group, but they answer different questions.

The T-score compares your bone density to the average peak bone mass of a healthy young adult of the same sex. Because bone density peaks in your late twenties and declines from there, most people over 50 will have a negative T-score. The World Health Organization set diagnostic thresholds based on this number in the early 1990s, and those categories are still in use:³

Normal: T-score of −1.0 or above
Osteopenia: T-score between −1.0 and −2.5, indicating lower-than-normal density that has not yet reached osteoporosis
Osteoporosis: T-score of −2.5 or below

The Z-score, by contrast, compares you to people your own age and sex. It is more useful for younger adults, premenopausal women, and children, where comparing to a 25-year-old reference would not make clinical sense. However, Z-scores should be interpreted carefully in men and women between 20 and 50, because how different DXA machines calculate them is not fully standardized.⁴

One common misunderstanding is treating the T-score as a direct measure of fracture risk. It is a significant risk factor, but not the only one. Clinicians often plug the T-score into a broader calculator called FRAX, which estimates your ten-year probability of a major osteoporotic fracture or hip fracture by combining bone density with clinical risk factors like age, smoking status, family history, and steroid use.⁵ A person with a T-score of −2.0 and several other risk factors may face a higher actual fracture probability than someone with a T-score of −2.6 and no other risk factors.

Who Should Be Screened

In the United States, the most widely cited guidelines come from the U.S. Preventive Services Task Force (USPSTF), which recommends screening for osteoporosis in all women aged 65 and older. For postmenopausal women younger than 65, screening is recommended when a clinical risk assessment indicates elevated fracture risk.⁶ Risk factors that might push screening earlier include a parent who fractured a hip, low body weight, smoking, and use of glucocorticoid medications.

For men, the evidence supporting routine screening is thinner. Most guidelines suggest considering DEXA scanning in men over 70 or in younger men with specific risk factors such as long-term steroid use, low testosterone, or heavy alcohol use. Men are often under-screened for osteoporosis relative to their actual fracture risk, partly because osteoporosis is still widely perceived as a condition that only affects women.

Beyond these standard recommendations, clinicians order DEXA scans for a range of other situations: monitoring bone health in people taking medications known to harm bone (certain cancer treatments, anticonvulsants, long-term proton pump inhibitors), evaluating unexplained fractures, and tracking the effectiveness of osteoporosis treatment over time.

How Much Radiation Are You Getting?

Very little. A standard DEXA scan of the spine and hip delivers a fraction of the radiation you receive from a chest X-ray. To put it in perspective, the worldwide average effective dose from natural background radiation is about 2.4 millisieverts (mSv) per year, which works out to roughly 6.6 microsieverts per day just from being alive on Earth.⁷ A DEXA scan typically delivers somewhere in the range of 1 to 10 microsieverts depending on the site scanned and the machine, which is comparable to one or two days of background exposure. This is one of the reasons the test can be repeated safely every year or two for monitoring.

Where DEXA Can Be Wrong

DEXA is the gold standard for bone density measurement, but it has blind spots. The most common source of error, especially in older adults, is degenerative changes in the spine. Bone spurs, disc calcification, and compression fractures can all make the spine appear denser than it actually is, falsely reassuring you that your bones are stronger than they are. A recent analysis found that degenerative or artifact-related errors appeared in about 16% of lumbar spine scans in older adults, compared to roughly 3% in younger people.⁸ Hip measurements are much less prone to this kind of artifact, which is one reason the hip is often the more reliable site in elderly patients.

Body size can also distort results. DEXA measures areal bone mineral density, meaning it calculates density based on the two-dimensional projection of a three-dimensional bone. A very small-framed person may register artificially low density simply because their bones are smaller, not because they are less mineralized per unit of volume. This becomes a major issue in children, as we will see below.

Another technology, quantitative computed tomography (QCT), can measure volumetric bone density and avoids some of these artifacts. In one study of 140 postmenopausal women, QCT detected osteoporosis in about 46% of patients while DEXA detected it in only 17%, largely because spinal degeneration was inflating the DEXA readings. Seven of the patients diagnosed with osteoporosis by QCT but not by DEXA had already experienced vertebral fractures.⁹ QCT is not routinely used for screening, though, because it delivers substantially more radiation and costs more. For most people, DEXA remains the right first-line test, as long as the interpreting physician accounts for potential artifacts.

Trabecular Bone Score

Bone density alone does not capture everything about bone strength. Two people with the same T-score can have very different fracture risk depending on the internal architecture of their bones, specifically the trabecular (spongy) mesh inside vertebrae and other bones. Trabecular bone score (TBS) is a relatively newer measurement derived from the same spine DEXA image you already get during a standard scan. It analyzes variations in the pixel pattern to estimate the quality of the trabecular microarchitecture without requiring any additional imaging.¹⁰

Both cross-sectional and long-term studies have shown that TBS can predict fragility fractures independently of bone mineral density, meaning it adds useful information on top of the T-score rather than just restating the same thing in different terms.¹¹ TBS is especially helpful in conditions where bone quality deteriorates faster than bone density drops, such as diabetes, glucocorticoid use, and other endocrine disorders. It can be incorporated into the FRAX calculator to refine fracture probability estimates. Not all DEXA facilities offer TBS analysis, but it is becoming more widely available as the software is a retrofit that runs on existing DXA hardware.

More Than Bones

DEXA technology is not limited to bone density. A whole-body scan can separate your body into three compartments: fat mass, lean mass (mostly muscle), and bone mineral mass, all in a single pass.¹² This makes DEXA one of the more accessible clinical tools for body composition analysis, and it is widely used in research settings, sports medicine, and increasingly in routine clinical care.

Beyond simple whole-body percent fat, DEXA body composition analysis can measure regional fat distribution, including estimates of visceral adipose tissue (the deep abdominal fat associated with metabolic disease) and the ratio of fat stored in the android (abdominal) region versus the gynoid (hip/thigh) region.¹³ These regional measures can tell a clinician more about metabolic risk than total body fat percentage alone.

One growing application is the assessment of sarcopenia, the age-related loss of muscle mass and function. By measuring appendicular lean mass (the lean tissue in your arms and legs) and adjusting for body size, DEXA can help distinguish between normal age-related changes and a more concerning degree of muscle loss.¹⁴ The combination of low bone density and low muscle mass, sometimes called osteosarcopenic obesity when excess fat is also present, is increasingly recognized as a particularly high-risk profile in older adults. DEXA is one of the few tools that can assess all three compartments in a single exam.

DEXA in Children and Adolescents

Interpreting DEXA scans in children requires a fundamentally different approach from adult scanning. Children and adolescents are still growing, and their bones are actively accruing mass, so comparing them to a peak adult reference (the T-score) makes no sense. Pediatric scans rely entirely on Z-scores, which compare a child to age- and sex-matched peers.¹⁵

The bone-size problem mentioned earlier is magnified in pediatric populations. DEXA measures areal density, not volumetric density, so a child who is short for their age will tend to look like they have lower bone density even if their bones are perfectly healthy for their size. This can lead to unnecessary alarm if the interpreting physician does not adjust for bone size or height. Guidelines for pediatric DXA interpretation emphasize accounting for growth status, skeletal maturity, and pubertal stage. The preferred scan site in children is typically the lumbar spine and whole body (minus the head), rather than the hip, because the hip’s growth plates complicate measurements during development.

DEXA scanning in children is not routine. It is reserved for situations where bone fragility is a real concern: conditions like osteogenesis imperfecta, chronic inflammatory diseases, prolonged steroid treatment, or a history of fractures that seem disproportionate to the level of trauma involved.

Transgender and Gender-Diverse Patients

Gender-affirming hormone therapy can affect bone density, and DEXA scanning is part of bone health monitoring for many transgender individuals. The interpretive challenge is which reference database to use when generating Z-scores and T-scores. The International Society for Clinical Densitometry (ISCD) issued guidance in 2019 recommending that Z-scores for transgender individuals be calculated using the reference data matching the person’s gender identity, rather than their sex recorded at birth. For gender-nonconforming individuals who do not identify within the binary, the society recommends using the reference data for the sex recorded at birth, though a provider can supply both male and female Z-scores if requested.¹⁶

For transgender patients aged 50 and older, a T-score of −2.5 or below is used for osteoporosis diagnosis regardless of hormonal status, the same threshold applied to cisgender patients. In younger transgender patients undergoing hormone therapy, interpretation can be more complex. A case series highlighted discrepancies in forearm bone density measurements in transgender individuals under 50 on gender-affirming hormones, with some showing scores suggestive of osteoporosis that raised questions about whether the standard reference databases were appropriate for their situation.¹⁷ The field is still working through how best to interpret DEXA results in populations whose skeletal development occurred under one hormonal environment and whose current hormonal environment is different.

Athletes and Relative Energy Deficiency

DEXA shows up frequently in sports medicine, both for body composition tracking and for bone health assessment. Athletes in weight-sensitive or endurance sports are at elevated risk for a condition now called Relative Energy Deficiency in Sport (REDs), where chronically low energy availability disrupts hormonal function and, over time, weakens bones. DEXA scanning of the lumbar spine and hip, with Z-score assessment, is a standard part of evaluating athletes suspected of having REDs.¹⁸

The tricky part is that athletes often have higher-than-average bone density in the sites that bear the load of their sport. A runner may have dense leg bones but weak lumbar vertebrae if their energy intake has been chronically low. DEXA can catch this regional mismatch. For athletes under 50, the Z-score is the relevant metric, and a Z-score below −1.0 at any site in an athlete raises a red flag, since active young people should generally have bone density at or above average for their age group.

Body composition analysis by DEXA has also become common in professional and collegiate sports settings, used to track lean mass changes during training seasons, monitor recovery from injury, and assess body fat levels. While convenient, this use is not without controversy: some sports medicine organizations have raised concerns about the pressure body composition numbers can place on athletes already vulnerable to disordered eating, and about the over-reliance on a single scan’s snapshot to make training or dietary decisions.

How Often to Repeat the Test

Bone density changes slowly. For most people, a follow-up DEXA scan every two years is sufficient to detect meaningful changes, and many clinicians stretch the interval to three to five years for patients whose initial results are normal and whose risk profile has not changed. More frequent scanning, sometimes annually, may be warranted if you have just started osteoporosis medication and your doctor wants to verify that it is working, or if you are on a medication known to cause rapid bone loss.

When comparing two DEXA scans over time, the change has to exceed a threshold called the least significant change before it is considered real rather than measurement noise. Each facility calculates this threshold based on its own equipment and technologist precision. A drop of one or two percent between scans does not necessarily mean your bones are getting weaker; it may simply reflect normal measurement variability. If your report shows a small change and your doctor is not concerned, that is usually why.

Consistency matters for serial monitoring. Ideally, you should use the same facility, the same machine, and even the same technologist for every scan. Switching between manufacturers’ devices can introduce apparent changes that are just differences in how the machines calibrate, not real shifts in your skeleton.