The National Institutes of Health provide a copy of the World Health Organization's definitions for bone density on its website. The definitions are based on the average bone density of a healthy 30-year-old adult.
A bone mineral density test is the best indicator for bone health. Several bone-related diseases can be determined from the test, and it can form the basis for as well as preventative measures. The test is performed by exposing the patient's bones to two beams of X-rays. The X-ray absorption of each bone in the patient's body is calculated and the results compared to the average density of all adults.