WHO Standards
The currently accepted conceptual definition of osteoporosis is that it is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fractures. Bone mineral can be measured with acceptable accuracy and precision and forms the basis for an operational definition of osteoporosis with better clinical utility. A study group of the World Health Organization (WHO) has proposed diagnostic guidelines for the interpretation of a bone mass measurement in Caucasian women. These guidelines are widely accepted as the industry standard when diagnosing osteoporosis:

Severe osteoporosis (or established osteoporosis)
A value for Bone Mineral Density (BMD) or Bone Mineral Content (BMC) more than 2.5 Standard Deviations (SD) below the young adult mean value in the presence of one or more fragility fractures.

Osteoporosis
A value for Bone Mineral Density (BMD) or Bone Mineral Content (BMC) that is more than 2.5 Standard Deviations (SD) below the young adult mean value.

A low bone mass (or osteopenia)
A value for Bone Mineral Density (BMD) or Bone Mineral Content (BMC) that lies between 1 and 2.5 Standard Deviations (SD) below the young adult mean value. Such individuals include those in whom the prevention of bone loss would be most useful.

Normal
A value for Bone Mineral Density (BMD) or Bone Mineral Content (BMC) that is not more than 1 Standard Deviations (SD) below the young adult mean value.