DEXA stands for "dual-energy x-ray absorptiometry," and is considered the most accurate test for bone density. Bone density tests are often used to screen for and detect the early stages of osteoporosis, a condition defined by a decreased density of normal bone. While standard x-rays show changes in bone density after about 40% of bone loss, a DEXA scan can detect changes after about a 1% change.
A DEXA scanner is a large machine that produces 2 x- ray beams, each with different energy levels. One beam is high energy while the other is low energy. The amount of x-rays that pass through the bone is measured for each beam. This will vary depending on the thickness of the bone. Based on the difference between the 2 beams, the bone density can be measured.
The results of a bone density measurement (DEXA scan) are reported in two ways: as T-scores and as Z-scores. A T-score compares your bone density to the optimal peak bone density for your gender. A Z-score is used to compare your results to others of your same age, weight, ethnicity, and gender. This is useful to determine if there is something unusual contributing to your bone loss.