Dual-energy X-ray Absorptiometry.

Bone mineral density (BMD) measurement by dual-energy X-ray absorptiometry (DXA) is the most commonly used method to assess fracture risk. DXA utilizes two different energy X-rays to calculate BMD and, by comparison to a young normative database, the T-score. In 1994, the World Health Organization defined osteoporosis based on T-score, changing the paradigm of the field and forever placing DXA measurements in the center of osteoporosis diagnosis. Since then, many large studies have demonstrated the predictive value of BMD by DXA-for every standard deviation decline in BMD, there is a relative risk of 1.5-2.5 for fracture. This predictive ability is similar to how blood pressure can predict myocardial infarction. Limitations of DXA are also important to consider. While BMD by DXA can identify those at risk, there is a significant overlap in the BMD of patients who will and will not fracture. Special considerations are also needed in men and ethnic minority groups. These groups may have different bone size, thus affecting the normative range of BMD, and/or distinct bone structure that affect the association between BMD and fractures. Finally, BMD can be affected by positioning errors or artifacts, including osteoarthritis, fracture, and jewelry. Of course, DXA has tremendous strengths as well-namely its wide availability, its low radiation exposure, and a large body of evidence that relate DXA measurements to fracture risk. For these reasons, DXA remains the cornerstone of fracture assessment now and for the foreseeable future.