Bone densitometry is the measurement of bone mineral density, almost always with a low-dose dual-energy X-ray scan, to judge how likely a bone is to break. The scan reports density in grams per square centimeter and converts it into a T-score and a Z-score, which place you against reference populations rather than an absolute standard. It is painless, takes ten to twenty minutes, and you stay fully clothed. Clinicians also call it a DXA scan or a bone density test, and the deeper reference on Bone densitometry covers who gets referred and why.
What it measures
Two X-ray beams of different energies pass through the body. Bone and soft tissue absorb them differently, so the software subtracts the soft tissue and solves for the mineral left in the beam path. The dose is below a standard chest X-ray.
The standard sites are the lumbar spine and the hip, with the femoral neck and total hip reported separately. The forearm is used when neither can be read, for example after joint replacement. Because the result is a flat projection rather than a true volume, body size affects it, and degenerative change, aortic calcification, collapsed vertebrae and metal hardware push the spine reading upward.
Two scores come out of the same figure. The T-score compares you with a healthy young adult reference; the Z-score compares you with people of your own age and sex, and is the one used in younger adults and children. Quantitative CT, heel ultrasound and peripheral devices also read bone, but the thresholds below apply only to central dual-energy scans.
Typical values
| T-score | Classification |
|---|---|
| -1.0 or higher | Normal bone density |
| Between -1.0 and -2.5 | Low bone mass |
| -2.5 or lower | Osteoporosis |
| -2.5 or lower with a fragility fracture | Severe, or established, osteoporosis |
These bands are the World Health Organization operational definition and apply to postmenopausal women and to men from about age 50. The middle band is covered separately under Osteopenia. In younger adults the Z-score is used, and -2.0 or lower is reported as below the expected range for age rather than as a disease label.
Machines from different manufacturers are calibrated differently, so values from two devices should not be compared. Bone changes slowly, so repeat scans are normally spaced a year or more apart. Who should be scanned, and from what age, differs between national guidelines, so treat any recommended age as one body’s position, not a universal rule.
Why it matters for longevity
Bone loss is silent until something breaks, so for most people a scan is the only way to learn that Osteoporosis has developed while there is still time to act. Lower density is associated with higher fracture risk across large cohorts, and the association steepens as the T-score falls.
The fracture is what changes a life, not the number. Cohort studies consistently report raised mortality in the year after a hip fracture, and a substantial chance of permanent loss of independence among those who survive it. Vertebral fractures are quieter, often never diagnosed, and accumulate into height loss, back pain and a stooped posture. The femoral neck value also feeds fracture risk calculators, which add age, sex, prior fracture, smoking and steroid use, because two people with the same score can carry very different risk.
What changes it
- Loading the skeleton. Resistance training and impact work such as jumping or brisk walking stimulate bone at the sites they load; swimming and cycling do not.
- Calcium, vitamin D and protein. Correcting a genuine shortfall helps; adding supplements to an adequate diet does much less.
- Medication. Long courses of oral glucocorticoids lower density. Bisphosphonates, denosumab and bone-forming agents raise it and have cut fracture rates in randomized trials.
- Hormonal change. Loss accelerates for several years around menopause as estrogen falls.
- Smoking, heavy drinking and low body weight. All work against bone, as does rapid weight loss without training or protein.
- Outside your control. Age, sex, genetics and the peak bone mass you built by your late twenties set the starting point.
Related reading
- How to Strengthen Bones and Prevent Osteoporosis
- Health Through Menopause: A Longevity Guide for Women
- Vitamin D and Longevity: Who Needs a Supplement
- DEXA
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.