This article forms part of our 'Tests and results' series for 2011 which aims to provide information about common tests that general practitioners order regularly. It considers areas such as indications, what to tell the patient, what the test can and cannot tell you, and interpretation of results. Dual energy X-ray absorptiometry (DXA) is the gold standard for diagnosing osteoporosis (OP). It uses X-rays at two energy levels and works on the principle that, as X-rays pass through body tissues they are attenuated to a different extent in different tissue types. The result - the bone mineral density (BMD) - can be reported at a number of sites. The most clinically useful are the lumbar spine, femoral neck and total hip (also termed the 'proximal femur').