DXA
Meaning of DXA: A quick low-dose scan that turns bone strength into a single comparable number, and the measurement osteoporosis is defined by.
Definition of DXA
DXA is a low-dose x-ray scan that measures how much mineral is packed into a given area of bone. Two x-ray beams of different energies pass through the body, and because bone and soft tissue absorb the two energies differently, the machine can separate them and report bone alone. It is the standard measurement used to identify osteoporosis and to follow bone density over time.
What does the scan actually measure?
Not the strength of a bone directly, which cannot be measured from outside, but the mineral content of a flat projection of it. The result is reported in grams per square centimeter, an area measurement rather than a true volume, which is why an unusually large or small skeleton can shift the number without the bone being abnormal. The dose involved is a small fraction of that from a chest x-ray, and the scan takes minutes with nothing entering the body.
What do the two scores mean?
| Score | Compared against | Used for |
|---|---|---|
| T-score | A healthy young adult of the same sex | Diagnosing osteoporosis after menopause and in older men |
| Z-score | Others of the same age, sex and size | Flagging bone loss with an underlying cause, and use in younger people |
Each score counts standard deviations from its reference, so a T-score of minus one sits one deviation below the young adult average. Osteoporosis is defined at a T-score of minus two and a half or lower, and the range between that and minus one is called low bone mass. The definition is circular by design: the disease threshold was set from the measurement, so the scan does not detect osteoporosis so much as define it.
Which sites are scanned?
The lower spine and the hip are the standard pair, because they contain the bone types that change earliest and they are the fractures that carry the most consequence. The forearm is used when neither is measurable, for instance after hip replacement. Scanning several sites and taking the lowest reading is standard, since loss does not proceed evenly.
What can distort the result?
Anything dense in the beam raises the apparent bone density without any real gain in strength: arthritic overgrowth in the spine, a healed compression fracture, calcium in the wall of the aorta, surgical metalwork, or a retained contrast agent from a recent test. Comparing scans done on different machines is unreliable for the same reason, so follow-up is normally done on the same device.
Also known as
Used in a sentence
A DXA scan of the lumbar spine and hip was arranged to quantify bone loss.
Good to know
The scan reports mineral per unit area rather than true volume, so body size, arthritic change, aortic calcification and surgical metal can all raise the number without any gain in bone strength.