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Your BMD 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.
The difference between your BMD and that of a healthy young adult is referred to as a standard deviation (SD).
The decision to test for BMD should be based on an individual's risk profile, and testing is never indicated unless the results could influence a treatment decision.