BMD
Meaning of BMD: The number a bone scan produces, describing how much mineral is packed into a measured slice of the skeleton.
Definition of BMD
BMD stands for bone mineral density, a measurement of how much mineral, mostly calcium salts, is packed into a given area of bone. It is obtained with a low-dose X-ray scan, usually of the hip and the lower spine. The result is reported not as a raw figure but as a comparison against the density expected in a healthy young adult.
How is it measured?
The standard method is dual-energy X-ray absorptiometry, usually shortened to DXA. Two X-ray beams of different energies pass through the body. Soft tissue and bone absorb the two beams to different degrees, so subtracting one measurement from the other isolates the mineral content. A bone mineral density scan takes a few minutes, involves lying still on a table, and delivers a radiation dose far below that of a standard chest X-ray.
What do the scores mean?
| Score | Compared against | What it is used for |
|---|---|---|
| T-score | A healthy young adult of the same sex | Defining osteoporosis and low bone mass |
| Z-score | Others of the same age, sex and size | Flagging a cause other than aging |
Both are expressed in standard deviations, a statistical measure of how far a value sits from the average. Fixed thresholds below zero separate normal density from low bone mass and from osteoporosis.
What lowers it?
Bone is living tissue, constantly broken down and rebuilt. Density falls whenever removal outpaces replacement. Age does this to everyone, and the drop accelerates in the years after menopause when estrogen withdrawal speeds up bone removal. Long courses of corticosteroids, low body weight, prolonged immobility, heavy alcohol use, smoking, overactive thyroid or parathyroid glands and some cancer treatments all push in the same direction. Low calcium or vitamin D intake limits the raw material available for rebuilding.
What does the result predict?
Density is one of the strongest single predictors of whether a bone will break, but it is not the only one. Age, a previous fracture, a parental hip fracture and tendency to fall all carry weight independently, which is why risk calculators combine the scan result with those factors rather than relying on the number alone.
Also known as
Used in a sentence
Repeat imaging showed a decline in BMD at the lumbar spine compared with the earlier study.
Good to know
Bone mineral density and bone mass are not the same quantity: density describes mineral within a measured area, while mass describes total amount, and a scan reports the former.