http://www.niams.nih.gov/Health_Info/Bone/Bone_Health/bone_mass_measure.asp
NIH Osteoporosis and Related Bone Diseases ~ National Resource Center
A bone mineral density (BMD) test is the best way to determine your bone health. BMD tests can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. This document provides information about the most widely recognized bone mineral density test, called a dual-energy x-ray absorptiometry or DXA test, and looks at bone mass measurements and what they mean.