Dual X-ray absorptiometry

Last edited 08/2020 and last reviewed 08/2022

This is the "gold standard" for measuring bone mineral density and it is widely considered to be the best means of assessing fracture risk.

• it is used to establish or confirm the diagnosis of osteoporosis, predict future fracture risk and for assessment of treatment efficacy by performing serial assessments (1)
• can be used to assess the bone mineral content of the whole skeleton or a specific site (2)
• most commonly measured sites are the lumbar spine (L1-L4) and at the femoral neck, where osteoporotic fractures are most likely to occur
• in elderly people with scoliosis, vertebral deformity, osteophytes or extraskeletal calcifications, the accuracy of measurement may be impaired. In these individuals the preferred site is the proximal femur (2)
• the BMD values are influenced by bone size as well as true density (2)

Similar technology has however been used, often in the context of research to assess bone loss at the wrist in conditions such as rheumatoid arthritis where peri-articular osteoporosis occurs.

Results are often quoted as a T score or Z score. This is somewhat confusing but it does allow the clinician to perform a limited assessment of an individual's risk of osteoporotic fracture taking other risk factors into account.

The World Health Organisation define osteoporosis as a bone mineral density of more than 2.5 standard deviations below the young adult mean (ie a t score < -2.5). This definition however encompasses a large proportion of the elderly population and as mentioned above it is not the only consideration in determining treatment.

Measurement of bone mineral density by DXA at the spine and hip should be carried out following fracture-risk assessment in patients in whom anti-osteoporosis treatment is being considered (3)

• note that there is variation in the utility of BMD at different sites with hip BMD performing better than spine BMD in predicting osteoporotic fractures.

Reference:

• (1) National Osteoporosis Foundation 2010. Clinician’s guide to prevention and treatment of osteoporosis  
• (2) 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)
• (3) SIGN (June 2020). Management of osteoporosis and the prevention of fragility fractures