Those results need different explanations. Reading them together is useful, but they should not be treated as interchangeable measures of health.
Start with the type of scan
DEXA, also written DXA, uses X rays to make measurements of bone and body tissues. A diagnostic bone density assessment commonly focuses on the hip and lumbar spine. A whole body composition scan estimates fat, lean tissue and bone mineral components.
A whole body figure is not a substitute for the appropriate site specific measurements used to assess osteoporosis. If your report is mainly about body composition, check whether diagnostic hip and spine measurements were actually performed.
Next, identify the date, the measurement sites and the report’s conclusion. For a repeat scan, check which previous examination is being used for comparison.
Bone mineral density is a measurement, not a percentage score
Bone mineral density, abbreviated BMD, is usually reported in g/cm² on a DXA bone density report. A T score and a Z score compare that measurement with a reference population.
The scores are expressed in standard deviations, which describe how far a result is from a reference average. A T score of −2.0 does not mean that you have lost 20% of your bone or that your bones are twice as weak.
This distinction matters because the same report can contain a BMD value, a reference score and a percentage comparison. They are different ways of describing information, not numbers that can be substituted for one another.
T scores and Z scores answer different questions
A T score compares the measurement with a young adult reference. It is generally the score used for bone density classification in postmenopausal women and men aged 50 and over.
A Z score compares the measurement with an age matched reference population, with other reference characteristics accounted for. It is generally preferred for women before menopause and men under 50. A Z score of −2.0 or lower is described as below the expected range for age.
A result within the expected range for age does not mean there are no bone health concerns. It answers a comparison question, not every question about fracture risk or the cause of a previous fracture. An older adult can have a low T score and a Z score closer to zero because the reference groups differ. The Z score does not cancel the T score.
Reading the T score categories
For people and measurement sites to which the standard classification applies:
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| T score | Bone density classification |
|---|---|
| −1.0 or higher | Normal range |
| Below −1.0 but above −2.5 | Low bone mass, often called osteopenia |
| −2.5 or lower | Osteoporosis range |
Classification is not the whole assessment. Fracture history and the broader medical picture remain important. These thresholds should not be applied indiscriminately to every age group or to every number on a whole body scan.
Consider this educational example: a postmenopausal woman has a valid total hip T score of −1.7 and a valid lumbar spine T score of −2.6. The lower valid spine result meets the bone density threshold for osteoporosis in this example. The two results should not be averaged into a milder category. The reporting clinician checks the validity of the sites and considers whether scan artefacts affect interpretation.
The useful question is: “Which measurements are valid for the conclusion, and how do they fit my fracture history?” It is not “Which single number looks most reassuring?”
Has bone density genuinely changed?
All measurement methods have some variation. The least significant change, or LSC, is a threshold calculated from the scanning facility’s precision assessment. It helps determine whether a difference is large enough to distinguish from measurement variability.
Here is a worked example:
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| Item | Example value |
|---|---|
| Earlier total hip BMD | 0.800 g/cm² |
| Later total hip BMD | 0.816 g/cm² |
| Numerical increase | 0.016 g/cm², or 2% |
| Facility’s LSC for this comparison | 3%, used here only as an example |
The numerical increase is below the example threshold. These scans therefore do not demonstrate a statistically significant increase using that criterion. This does not establish that treatment has failed; it means the apparent increase cannot confidently be separated from measurement variation.
The 3% value is not a universal threshold. Your facility should use its own appropriate precision information. The comparison also needs compatible measurements. Different machines require appropriate cross calibration before their results can be compared quantitatively.
Compare the actual BMD at the relevant site, not a percentage change calculated from a T score. Ask whether the report states that the change exceeds the LSC.
Body composition: read the amount as well as the percentage
Body fat percentage is the proportion of the measured mass attributed to fat. Fat mass describes the amount. Lean tissue is the nonfat soft tissue component and includes more than skeletal muscle.
Here is a simplified arithmetic example, not a DXA classification:
A person with a total mass of 80 kg and 24 kg of fat has 30% body fat. Another person with a total mass of 100 kg and 30 kg of fat also has 30% body fat. The same percentage does not mean the same amount of fat or the same physical capacity.
Within one person, a percentage can also change because more than one component has changed. Read fat mass, lean tissue and the conditions of the scan rather than judging the result from the percentage alone.
Lean tissue is not the same as muscle strength
A scan does not measure how much force you can produce. It also cannot turn a change in lean tissue into proof that you gained exactly that amount of working muscle.
Fluid balance, recent exercise, food intake and aspects of scan technique can affect body composition measurements. Research on DXA methodology therefore emphasises consistent preparation, positioning and analysis.
Follow the provider’s preparation instructions rather than imposing your own fasting or fluid restriction. For a meaningful repeat comparison, ask what needs to remain consistent and tell the provider when the conditions differ.
Strength measures, daily function and training performance provide different information from the scan. Together they can help you assess whether a body composition goal is being pursued in a useful way.
Turn the report into a small set of questions
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| What you are reading | What to ask |
|---|---|
| Bone density conclusion | How does this fit my age, fracture history and other risk factors? |
| T or Z score | Is this the appropriate score and reference for me? |
| Repeat measurement | Was the comparison valid, and did it exceed the facility’s LSC? |
| Body composition change | Which components changed, and were the scan conditions comparable? |
| Recommendation | What action is appropriate, and when would another scan change care? |
You do not need to solve every technical issue yourself. These questions help you have a more useful discussion with the reporting clinician and your treating team.
What changes in an exercise plan?
A bone health finding may influence the choice and progression of resistance, impact and balance exercise. The relevant fracture history and medical advice matter alongside the score.
A body composition goal may lead to tracking strength and function as well as changes in fat or lean tissue. Nutrition support may also be appropriate. A report is not a reason to pursue a universal body fat target or repeat scans frequently without a clear purpose.
At Longevico, relevant findings can inform the exercise plan, with medical interpretation and treatment decisions kept with the appropriate clinician. A full Health Review is a separate service, not a prerequisite for discussing an existing report in a focused assessment.
Explore the Clinical Exercise Assessment. Bring the complete report through the agreed secure process, including the written conclusion and previous comparison where available.
Related reading
Exercise for osteopenia and osteoporosis · Strength and fitness after 40
Evidence and further reading
- International Society for Clinical Densitometry. Official Adult Positions. 2023. Bone density interpretation, follow-up measurement, precision and body composition. Read source
- US National Institute of Arthritis and Musculoskeletal and Skin Diseases. Bone Mineral Density Tests: What the Numbers Mean. 2023. Read source
- International Society for Clinical Densitometry. Precision Assessment & Calculator FAQs. Current technical guidance. Read source
- American College of Radiology and Radiological Society of North America. Bone Densitometry (DEXA, DXA). RadiologyInfo.org. Read source
- Nana A, Slater GJ, Stewart AD, Burke LM. Methodology Review: Using Dual-Energy X-Ray Absorptiometry (DXA) for the Assessment of Body Composition in Athletes and Active People. International Journal of Sport Nutrition and Exercise Metabolism. 2015;25(2):198–215. doi:10.1123/ijsnem.2013-0228. Read source
- Royal Australian College of General Practitioners and Healthy Bones Australia. Osteoporosis management and fracture prevention in postmenopausal women and men over 50 years of age. Second edition. 2024. Exercise chapter. Read source