Bone mass or bone mineral density (BMD) refers to grams of bone mineral — specifically hydroxyapatite — per square centimeter of bone cross-section and is expressed in units of g/cm2. However, there is a limitation as BMD is a two-dimensional image of a three-dimensional object.
T-Score and Z-Score
A patient's BMD, measured by densitometry and expressed in g/cm2, is compared with a "normal value." The normal value is the mean BMD of sex-matched young adults at their peak bone mass, sometimes referred to as the "young adult mean." When compared with the normal value, a patient's BMD can be expressed in terms of the number of standard deviations (SD) from the normal value. A convenient way to express this is a T-score. The Z-score compares a patient's BMD in a different way, it is matched to individuals of the same age and sex.
BMD or bone mineral content (BMC) not more than 1 SD below the young adult mean (T-score above -1).
Osteopenia (or low bone mass):
or BMC between 1 and 2.5 SD below young adult mean (T-score between -1 and -2.5).
BMD or BMC 2.5 SD or more below the young adult mean (T-score at or below -2.5).
Severe osteoporosis (or established osteoporosis): BMD or BMC 2.5 SD or more below the young adult mean in the presence of one or more fragility fractures.