DXA

Dual-energy X-ray absorptiometry (DXA) is a medical imaging method used to assess bone mineral density (BMD) and, with specialized software, body composition. It employs two X-ray beams at different energy levels to distinguish bone from soft tissue, allowing the calculation of areal BMD in grams per square centimeter at specific skeletal sites.

DXA measurements are most commonly taken at the lumbar spine and hip, with peripheral sites such as the forearm or heel also used in some settings. The resulting BMD value is interpreted using T-scores and Z-scores. A T-score compares a person’s BMD to a young healthy reference population and is used to diagnose osteoporosis in older adults, with a score of -2.5 or lower constituting osteoporosis. A Z-score compares BMD to age-, sex-, and ethnicity-matched norms and is typically used in younger individuals or for investigating secondary causes of low bone density. Some DXA systems also estimate regional body composition, including fat mass and lean mass.

Indications for DXA include assessing fracture risk, diagnosing osteoporosis or osteopenia, and monitoring response to osteoporosis therapy. Central DXA ( spine and hip ) is generally preferred for BMD assessment, while peripheral DXA can be useful for screening when central DXA is unavailable. Results may be influenced by factors such as degenerative spinal changes, vascular calcifications, recent fractures, or differences between device manufacturers, necessitating careful interpretation and, if needed, cross-calibration.

DXA delivers a very low radiation dose and is considered safe for routine use, though it provides information about bone density rather than bone quality or fracture risk alone.