Your BMD (bone mineral density) is compared to two norms, "young normal" and "age-matched." Young normal, known as your T-score, compares your BMD to optimal or peak density of a 30-year old healthy adult and determines your fracture risk, which increases as BMD falls below young-normal levels. Age-matched, known as your Z-score, compares your BMD to what is expected in someone your age and body size. Among older adults, however, low BMD is common, so comparison with age-matched norms can be misleading. 

The difference between your BMD and that of a healthy young adult is referred to as a standard deviation (SD). As outlined in the World Health Organization's diagnostic categories, individuals whose T-score is within one standard deviation of the "norm" are considered to have normal bone density. Scores below the "norm" are indicated in negative numbers. For example, a score from -1 to -2.5 SD below the norm indicates low bone mass, or osteopenia, and a score of more than -2.5 SD below the norm is considered a diagnosis of osteoporosis. For most BMD tests, -1 SD equals a 10-12 percent decrease in bone density.

In summary:

• T score greater than or equal to -1.0---Normal bone density
• T score less than -1.0 but greater than -2.4---Osteopenia
• T score less than or equal to -2.5---Osteoporosis 

Z-scores are important to see if your values are similar to women your own age. If your Z score is abnormal, it suggests that other factors besides age may be responsible for your bone loss. A laboratory workup may be done to determine this.