DEXA
Meaning of DEXA: The abbreviation is also written DXA, and the two forms name the same low-dose x-ray method for measuring bone mineral density.
Definition of DEXA
DEXA stands for dual energy x-ray absorptiometry, a method of measuring how much mineral a given area of bone contains. Two x-ray beams of different energies pass through the body, and because bone and soft tissue absorb the two energies by different amounts, the mineral in the bone can be calculated separately from the tissue surrounding it.
Why are two x-ray energies needed?
A single x-ray beam is weakened by everything it passes through, so bone and the fat and muscle around it are mixed together in the result. Using two beams of different energy solves that. Each tissue weakens the two beams in its own characteristic ratio, so the machine can subtract the soft tissue mathematically and report what the bone alone contains. The result is an areal density, mineral per unit of surface area rather than per unit of volume, which means a physically larger bone tends to read slightly higher even at the same true density. The same two-energy trick is also used to measure body composition, separating fat from lean tissue and bone.
What do the scores mean?
Raw density values are hard to interpret on their own, so results are converted into standardized scores that say how far a measurement sits from a reference group.
| Score | Compared against | Mainly used for |
|---|---|---|
| T-score | The average peak density of healthy young adults of the same sex | Classifying osteoporosis and low bone mass in older adults |
| Z-score | People of the same age, sex and often similar body size | Children, younger adults, and looking for a secondary cause |
The distinction is commonly misunderstood. The T-score, which is the one used to define osteoporosis, does not adjust for age at all: it compares an older adult against young adults at their peak. The Z-score is the age-matched figure, and it is used when the question is whether density is unusual for that person rather than lower than a youthful peak.
Where on the body is it measured?
The hip and the lower spine are the usual sites, because fractures there carry the greatest consequences and the measurements there predict future fracture best. A forearm site is used when the hip or spine cannot be assessed, for instance after joint replacement or where spinal arthritis falsely raises the reading. Serial testing tracks change in bone density over time, and a DEXA scan repeated on the same machine is far more comparable than one repeated elsewhere, since small differences between scanners can exceed real biological change.
Also known as
Used in a sentence
The patient's DEXA results showed bone density within the expected range for age.
Good to know
DEXA results are reported as T-scores and Z-scores rather than raw density, and the two answer different questions: the T-score compares a person with healthy young adults at peak bone mass, while the Z-score compares them with others of the same age.