A bone density test determines if you have osteoporosis - a disease that causes bones to become more fragile and more likely to break.In the past, osteoporosis could be detected only after you broke a bone. By that time, however, your bones could be quite weak. A bone density test enhances the accuracy of calculating your risk of breaking bones.A bone density test uses X-rays to measure how many grams of calcium and other bone minerals are packed into a segment of bone. The bones that are most commonly tested are in the spine, hip and forearm.