bone mineral density (BMD)

Last reviewed 01/2018

Bone mineral density (BMD) is the amount of bone mass per unit volume (volumetric density) or per unit area (areal density) (1).

  • areal BMD is expressed either
    • in grams of mineral per square centimeter scanned (g/cm2) or
    • as a relationship to two norms - Z-score (compared to the expected BMD for the patient's age and sex) or T-score (compared to "young normal" adults of the same sex)
    • the difference between the patient's score and the norm is given as standard deviation (SD) above or below a mean
    • normally 1 SD equals to 10-15% of the BMD value in g/cm2 (2)

A decline in BMD (depending on the skeletal site) starts in the young adulthood which accelerates at the time of menopause and continues to progress in postmenopausal women and men age 50 and older (2).

The world health organization (WHO) has published the following diagnostic classification based on BMD measurements at the spine, hip or forearm by DXA devices.

  • normal
    • BMD is within 1 SD of a "young normal" adult (T score at -1 and above)
  • low bone mass
    • BMD is within 1 and 2.5 SD below that of a "young normal" adult (T score between -1 and -2.5)
  • osteoporosis
    • BMD is 2.5 SD or more below that of a "young normal" adult (T score at or below -2.5)
    • above patients who have already experienced one or more fractures are categorized as severe or "established" osteoporosis (2)


  • in premenopausal women and men who are less than 50 years of age along with children, the WHO BMD diagnostic classification should not be applied (2)


  • 1. International Osteoporosis Foundation and National Osteoporosis Foundation 2011. Osteoporosis: burden, health care provision and opportunities in the EU. A report prepared in collaboration with the International Osteoporosis Foundation (IOF) and the European Federation of Pharmaceutical Industry Associations (EFPIA)
  • 2. National Osteoporosis Foundation 2010. Clinician's guide to prevention and treatment of osteoporosis