Bone density test results of '2.5' or lower are indicative of osteoporosis. This means that the mass of calcium and minerals in the bones is at least 2.5 standard deviations below the mean of young, healthy adults.
The tests measures bone health relative to the established norms of a healthy, 30-year old adult. A score of '0' is exactly equal to that of a young, healthy adult, though scores from '+1' to '-1' are considered normal -one standard deviation above or below. Scores ranging from '-1' to '-2.5' indicate the individual has low bone mass, while those '-2.5' or lower demonstrate an increased risk of fracture.