When your DEXA report comes back, you will almost always see two numbers: a T-score and a Z-score. These are not measurements of how strong your bones are in absolute terms — they are comparisons against a reference group. Understanding what they compare to (and what the numbers actually predict) is the most useful thing you can take away from your DEXA scan.
What is a T-score?
A T-score compares measured bone mineral density with a young-adult reference database. For diagnostic classification, the ISCD recommends the young adult White female NHANES III reference for femoral-neck T-scores in both women and men. It is a comparison—not a direct record of your own lifetime peak.
The T-score is reported in standard deviations:
- 0 means your bone density matches that of a healthy young adult.
- Negative numbers mean your bones are less dense than that reference.
- Positive numbers mean your bones are denser than that reference.
How to read a T-score
For postmenopausal women and men aged 50 or older, central DXA T-scores are commonly classified as:
- T-score above −1.0 → Normal bone density.
- T-score between −1.0 and −2.5 → Osteopenia (low bone density, increased fracture risk).
- T-score of −2.5 or lower → Osteoporosis.
- T-score of −2.5 or lower plus a fragility fracture → Severe (established) osteoporosis.
Fracture risk rises as BMD falls, but no fixed multiplier applies to every age, site, and patient. Clinical risk factors can matter as much as the score.
T-score is for postmenopausal women and men 50+
The T-score categories above were developed specifically for postmenopausal women and men aged 50 and over. They are not appropriate for younger adults, premenopausal women, or children — in those groups, the Z-score is the right tool.
What is a Z-score?
A Z-score compares your bone density to the average for people of your same age, sex, and ethnic background. It answers a different question: "is my bone density typical for someone like me right now?"
- Z-score above −2.0 → typical for your age group.
- Z-score of −2.0 or lower → bone density is lower than expected for your age and warrants a search for an underlying cause.
A low Z-score in a younger person is a flag. It suggests something specific may be driving bone loss — a hormonal condition, a chronic illness, a medication (often long-term steroids), nutritional deficiency, or excessive alcohol intake.
When to focus on T-score versus Z-score
- Postmenopausal women and men 50+ → use the T-score for diagnostic classification, then combine it with fracture history and clinical risk to decide treatment.
- Premenopausal women, men under 50, and children → focus on the Z-score. The T-score is misleading at these ages.
- Anyone with a low Z-score → workup for a secondary cause of bone loss, regardless of T-score.
What about the BMD number itself?
Your report will also show a raw bone mineral density (BMD) value in grams per square centimetre (g/cm²) for each site scanned (lumbar spine, hips, sometimes forearm). The T- and Z-scores are calculated from this raw number.
The raw BMD matters most when you are being monitored over time — small changes in BMD between scans are how doctors track whether treatment is working. The T- and Z-scores matter most for first-time interpretation.
Why are different sites reported separately?
A DEXA scan usually measures the lumbar spine and both hips. Different sites can give different results because bone loss is not always even across the body:
- The spine is dominated by trabecular (sponge-like) bone, which is more metabolically active and changes faster.
- The hip is a mix of trabecular and cortical (dense outer) bone, which changes more slowly.
- Diagnostic classification uses the lowest valid T-score from the lumbar spine, total hip, or femoral neck; the one-third radius is used in specified circumstances. Artefactual or excluded regions should not drive the diagnosis.
What if my T-score and Z-score disagree?
This happens all the time, especially in younger or premenopausal patients. A T-score in osteoporosis range with a normal Z-score usually means "your bones are low compared to a 30-year-old, but normal for your age and stage of life." Your doctor will weigh both, plus your overall fracture risk, to decide what (if anything) needs to be done.
Can I make my T-score go up?
Sometimes, but treatment success is not defined by making the T-score rise. Stable BMD without a new fracture may be a satisfactory response, and a change smaller than the facility's LSC may be measurement noise. Exercise, nutrition, smoking cessation, falls prevention, and prescribed medication address different parts of fracture risk.
What do I actually do with my numbers?
- T-score above −1.0 → continue routine bone-healthy habits.
- T-score −1.0 to −2.5 (low bone mass) → combine the score with fracture history and validated risk assessment; medication and repeat timing are individual decisions.
- T-score −2.5 or lower (osteoporosis range) → discuss secondary causes and treatment; repeat timing depends on whether it will change management.
- Low Z-score → workup for an underlying cause.
What are the important limitations?
DXA measures two-dimensional areal bone mineral density; it does not measure every aspect of bone strength. Arthritis, vertebral compression, calcification, previous surgery, metalwork, positioning, and differences between machines can distort results. Fracture risk must combine the usable DXA sites with age, previous fractures, medicines, falls, medical conditions, and—when appropriate—a validated risk tool.
A fragility fracture can justify an osteoporosis diagnosis or treatment assessment even when the T-score is not in the osteoporosis range. For monitoring, compare valid studies from the same facility and machine where possible, and only call a change real when it exceeds that facility's least significant change (LSC). The timing of another scan should be individualised.
Questions to ask your care team
- Which sites were usable, and did arthritis, fracture, surgery, or metal affect the measurement?
- Should my report be interpreted using T-scores, Z-scores, fracture history, or a risk calculator?
- If this is a repeat study, was it compared with the correct baseline and did the change exceed the facility's LSC?
- Does the result change treatment now, and what clinical reason determines the timing of any repeat DXA?
Sources and further reading
- International Society for Clinical Densitometry: 2023 Adult Official Positions
- RadiologyInfo.org: Bone density scan (DXA)
Conclusion
Your T-score compares BMD with a specified young-adult reference, while your Z-score compares it with an age- and sex-matched reference. Neither directly measures bone strength. Use the valid score for your age and menopausal status alongside fracture history, artefacts, and overall clinical risk.