The bone density test (DXA) explained: scores, limits and preparation
What a DXA scan measures, what a T-score or Z-score means, who the numbers apply to, and what the test cannot tell you on its own.
You have been offered a bone density scan, or you have a report with numbers like "-1.8" on it and no clear idea what they mean. This page explains what the test measures, how the two main scores work, how often scans are repeated according to different sources, and what to expect on the day. It sits alongside our main guide to osteoporosis and bone health. Nothing here interprets a personal result, and your own doctor has the final word on what your numbers mean.
What a DXA scan measures
The most common bone density test is called dual-energy X-ray absorptiometry, shortened to DXA or DEXA. The U.S. National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) explains that it uses radiation to measure how much calcium and other minerals are in a specific area of your bone. Hip and spine are usually the sites measured, because those are the places where fractures do the most harm.
The result is bone mineral density, or BMD. It is a measure of how much mineral is packed into a given area of bone. The number is one piece of information, and the sections below explain why it is not the whole picture.
T-scores and Z-scores in plain language
A DXA report gives your density as a score that compares you with a reference group. Which score you receive depends on your age and sex.
A T-score compares your bone density with that of a healthy young adult. NIAMS says it is the difference between your bone mineral density and 0, which is the bone mineral density of a healthy young adult. It is the score used for postmenopausal women and for men aged 50 or older. The lower the T-score, the higher the fracture risk. NIAMS gives a sense of scale: fracture risk goes up by 1.5 to 2 times with each 1-point drop in the T-score.
A Z-score compares you with the average for people of your own age, sex and ethnicity. NIAMS says it is used for premenopausal women, men younger than 50, and children. A Z-score of -2.0 or less is described there as low bone density. A low Z-score can prompt a doctor to look for a cause outside normal aging, such as a medicine or another condition.
T-score categories
The International Osteoporosis Foundation notes that the World Health Organization defined the threshold values used for DXA. The categories below are as the Foundation and NIAMS state them, for the hip, in postmenopausal women and men aged 50 or older.
| T-score | Category |
|---|---|
| -1 and above | Normal |
| Between -1 and -2.5 | Osteopenia (low bone mass) |
| -2.5 or lower | Osteoporosis |
| -2.5 or lower, with at least one fragility fracture | Severe osteoporosis |
The foundation's patient page adds that these categories apply to hip bone density. The Bone Health and Osteoporosis Foundation (BHOF) also notes that osteoporosis can be diagnosed with a T-score above -2.5 after a spine or hip fracture, or when a risk calculator shows high risk. In other words, the table is a guide, and clinicians weigh other factors too.
Who the score applies to
T-score categories were built for postmenopausal women and men aged 50 and older. They are not meant to label younger adults, which is why premenopausal women and younger men receive Z-scores. Children are also given Z-scores.
The BHOF adds that a low result does not always mean osteoporosis. It notes that a low T-score can sometimes reflect another condition, such as osteomalacia or multiple myeloma, so a clinician confirms the diagnosis. Hormone-related and medicine-related causes are touched on in the main osteoporosis guide, and calcium regulation is covered in the parathyroid glands and calcium.
Who is usually tested
Recommendations differ between groups and countries. The U.S. Preventive Services Task Force (USPSTF) recommends screening with DXA for women 65 years or older, and for postmenopausal women younger than 65 who are at increased risk by a clinical risk assessment. For men, it found the evidence insufficient to recommend for or against routine screening.
The BHOF lists a wider group. It says women 65 and older, men 70 and older, and anyone who has broken a bone after age 50 should be tested, along with women aged 50 to 64 and men aged 50 to 69 who have risk factors. A person with a medical condition or medicine that affects bone may be tested earlier. If you live outside the United States, a national guideline may draw the lines differently.
How often scans are repeated
There is no single answer. Sources disagree because the right interval depends on the person's risk and on whether treatment has started.
The BHOF says repeat testing is often done 1 to 2 years after starting or changing an osteoporosis medicine, to check the response, and that timing otherwise depends on the individual situation. The patient information site RadiologyInfo.org says routine evaluations every two years may be needed to see a significant change in bone mineral density, and that people on high-dose steroids may need follow-up at six months. The USPSTF, on the other hand, states no screening interval, and cites evidence that repeating BMD testing at an interval of 4 to 8 years did not improve accuracy in predicting fractures.
Because results from different machines cannot be compared directly, RadiologyInfo.org advises that follow-up scans ideally use the same machine at the same facility.
Limits, and other tools used alongside DXA
RadiologyInfo.org states plainly that DXA cannot predict who will fracture. It gives relative risk and helps decide whether treatment is needed. It also notes that vertebral compression fractures or osteoarthritis may interfere with the accuracy of the test, and that spinal deformity or earlier spinal surgery limits its usefulness.
That is where risk calculators come in. The International Osteoporosis Foundation says DXA's limitations led to the development of FRAX, a calculator that combines bone density with other risk factors. The USPSTF lists FRAX and other calculators, and notes that FRAX has no defined screening threshold. Some centers also offer add-on measurements, and the BHOF mentions vertebral fracture assessment and trabecular bone score. Heel or finger tests and ultrasound are mainly screening tools; the BHOF states they cannot accurately diagnose osteoporosis and should not be used to see how well a medicine is working.
Preparing for the scan
Preparation is simple. RadiologyInfo.org says you should not take calcium supplements for at least 24 hours before the exam, should wear loose clothing without metal zippers, belts or buttons, and should leave jewelry and other metal items behind. If you recently had a barium exam or a contrast injection for a CT or radioisotope scan, you may need to wait 10 to 14 days. Women should tell the technologist if they might be pregnant. Follow the instructions from the imaging center, since they can differ.
The scan itself is painless. You lie on a table while an arm passes over you. RadiologyInfo.org says it is usually completed within 10 to 30 minutes, while the BHOF describes it as taking only a few minutes, so the length depends on the equipment and the areas scanned. Most people need a referral. The results usually go to the ordering clinician, who explains them. A list of useful questions for that conversation is in questions to ask an endocrinologist.
About the radiation
Radiation is a fair concern, and the sources are reassuring. RadiologyInfo.org says the dose is less than one-tenth the dose of a standard chest X-ray. The BHOF says the exposure is very low, far less than a cross-country flight. Doses vary between machines. If you are pregnant or think you might be, say so before the scan.
Frequently asked questions
What is a good T-score on a bone density test?
Sources define -1 and above as normal bone density for postmenopausal women and men over 50. A score between -1 and -2.5 is called osteopenia, and -2.5 or lower is the osteoporosis range. Whether a particular score matters for you depends on your age and other risk factors.
What is the difference between a T-score and a Z-score?
A T-score compares you with a healthy young adult, while a Z-score compares you with people of your own age, sex and ethnicity. A very low Z-score can suggest that something besides aging is involved.
Does a bone density test hurt?
No. The sources describe it as painless and non-invasive. You lie still on a table, usually fully clothed apart from metal items.
Can I get a bone density test without a doctor's referral?
The BHOF says most people need a prescription or referral. Arrangements vary by country and by health system, so the ordering route is worth checking locally.
The short version
A DXA scan measures mineral density at the hip and spine and reports it as a T-score or a Z-score depending on age and sex. The categories are useful, but they describe risk and do not predict a fracture, and how often to repeat the test varies by source and by person. Your own doctor or specialist is the right person to put the number in context.