The WHO Working Group defines osteoporosis according to measurements of bone mineral density (BMD) using dual-energy X-ray absorptiometry (DEXA).² Thus osteoporosis is defined as a bone density T score at or below 2.5 standard deviations (T score) below normal peak values for young adults.

Established or severe osteoporosis is present when there is at least one or more fragility fracture in conjunction with a T score < -2.5. Osteopaenia refers to T scores between –1.0 and –2.5. Normal bone density is present if the T score is greater than –1.² These criteria were initially established for the assessment of osteoporosis in Caucasian women. BMD reports may include a “Z score” which is the number of standard deviations by which the subject of interest differs from the mean for their age. This is of greater clinical utility in younger individuals.

DEXA BMD Values	Definition
T score > -1.0 S.D	Normal bone mineral density
T score between –1.0 and –2.5 SD	Osteopaenia
T score < - 2.5 SD	Osteoporosis
T score < - 2.5 SD with 1 or more fragility fractures	Severe osteoporosis

The WHO definition of osteoporosis only takes into consideration measurement of bone density, with no component of bone quality.

A clinical definition of osteoporosis was developed in 2001 by the NIH Consensus Development Panel on Osteoporosis. It stated: “Osteoporosis is defined as a skeletal disorder characterised by compromised bone strength predisposing a person to an increased risk of fracture”. This definition takes into consideration that there are other factors that influence bone quality such as the microarchitecture of bone.² However, measurement of BMD remains the most useful clinical tool available for diagnosing osteoporosis.

It is also important to take into consideration risk factors for osteoporosis in addition to measured bone mineral density, when making decisions about commencing treatment.

Interpretation of DEXA Measures

Content updated July 28, 2009