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 your bone mineral density to the average bone density of a healthy young adult (around age 30) of your same sex. It tells you how much your bones have changed from their 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
The World Health Organization defines clear categories based on T-score:
- 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.
A useful rough rule: every 1.0 drop in T-score roughly doubles your fracture risk.
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+ → focus on the T-score to decide on osteoporosis 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.
- Your overall diagnosis is based on the lowest T-score among the measured sites — so if your spine is normal but your hip is in osteoporosis range, the diagnosis is osteoporosis.
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?
Yes, slowly. Bone density responds to weight-bearing exercise, adequate calcium and vitamin D intake, stopping smoking, limiting alcohol, and — when prescribed — osteoporosis medications. Most patients on treatment see a measurable improvement in T-score over 1 to 2 years.
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 (osteopenia) → lifestyle measures, sometimes calcium and vitamin D, and a repeat DEXA in 2 to 5 years depending on risk.
- T-score below −2.5 (osteoporosis) → talk to your doctor about treatment. Repeat DEXA in 1 to 2 years to monitor response.
- Low Z-score → workup for an underlying cause.
Conclusion
Your T-score tells you how your bones compare to their lifetime peak; your Z-score tells you how they compare to others your age. The T-score drives osteoporosis diagnosis in older adults; the Z-score is the right number for younger people and a flag for secondary causes at any age. Both numbers, read alongside your overall fracture risk, are what guide the conversation with your doctor about what to do next.