DEXA stands for "dual-energy x-ray absorptiometry" and BMD stands for Bone Mineral Density DEXA is considered the most accurate test for bone density. 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.
DEXA uses a small amount of radiation. However, the amount of radiation used in a DEXA scan is very low, less than the amount of radiation you get in one day just living on Earth.
Osteoporosis is a common bone disease that causes bone to become weak. This weakness can lead to fractures of the spine, hip, ankle, and wrist from simple falls which might not have resulted in a fracture in someone with normal bones. The World Health Organization is the source of the commonly accepted definitions for osteoporosis (WHO Technical Report Series #843, Geneva 1994):
Normal: A value for BMD statistically within 1 standard deviation of the young adult peak bone mass. The report shows a T score between +1 and -1, signifying a BMD within the normal range.
Low bone mass (medically termed Osteopenia): A value for BMD greater than 1 standard deviation but less than 2.5 standard deviations below that of an average young adult. The report shows a T score between -1 and -2.5, which signifies an increased fracture risk but does not meet the criteria for osteoporosis.
Osteoporosis: A value for BMD 2.5 or greater standard deviations below that of the average peak young adult bone mass. BMD in this range signifies an even higher fracture risk than osteopenia. The report shows a T score of -2.5 or lower.