A bone density report contains two numbers derived from the same measurement. They differ only in what population the result is compared against, and that choice determines what the number is capable of telling a clinician.
What the scan physically measures
Dual-energy X-ray absorptiometry passes two X-ray energies through a region of bone and calculates how much each is attenuated, since bone and soft tissue absorb the two energies differently.
The output is areal bone mineral density, expressed as mineral content per unit of projected area rather than per unit of volume.
Because it is areal rather than volumetric, the measurement is influenced by bone size, which is one of several reasons raw density values are not compared directly between people.
The T-score uses a young adult reference
A T-score expresses how far the measured density sits from the mean of a healthy young adult reference population, counted in standard deviations.
Diagnostic thresholds for osteoporosis and low bone mass in postmenopausal women were defined against this reference, which is why the T-score is the score used for that classification.
The logic is that peak bone mass represents the reference state, so the T-score describes how much has been lost from that peak rather than how a person compares to others their age.
The Z-score compares to age-matched peers
A Z-score compares the same measurement against people of similar age, sex and often body size.
Because bone density declines with age in everyone, an older adult can have a concerning T-score while holding an unremarkable Z-score, and both statements are accurate.
A markedly low Z-score suggests something beyond expected age-related change, which is why it is the score emphasized in premenopausal women, younger men and children.
Site and technique affect the result
Scans typically measure the lumbar spine and hip, and results at the two sites can diverge because they contain different proportions of trabecular and cortical bone.
Degenerative changes, spinal hardware and vascular calcification can artificially raise measured spine density, so a report often excludes affected vertebrae.
Comparing scans over time is only valid on the same machine or with cross-calibration, since equipment differences exceed the small changes being tracked.
Density is one input among several
Fracture risk depends on bone quality, geometry and fall risk in addition to density, so a scan does not by itself determine risk.
Clinical risk assessment tools combine density with factors such as age, prior fracture and certain medications to estimate probability over a defined horizon.
Interpreting these results, and deciding whether and when to rescan, is a clinical judgment made with a physician rather than read off the report.