What does a DEXA scan tell you that a regular bone density measurement does not?

A regular X-ray shows what bones look like, but provides no number for how dense and solid they are. Bone loss only becomes visible on such an image once roughly 30% of bone mass has already disappeared. DEXA measures bone mineral density quantitatively at the sites that matter most: the spine, the hip, and optionally the whole body. This allows it to detect bone loss at an early stage, long before a standard X-ray reveals anything.

The WHO uses a T-score of -2.5 or lower on a DEXA scan as the official threshold for a diagnosis of osteoporosis. That T-score compares your bone density with that of healthy young adults. A standard X-ray yields no T-score, making standardised diagnosis and treatment decisions impossible. Of 52 national guidelines across 36 countries, more than half incorporated a fracture-risk calculation based in part on DEXA data.

Older methods could only measure bone density at the wrist or heel, the peripheral extremities of the skeleton. DEXA can image the spine from the front and the side, the hip, and the whole body; it offers better precision, stable calibration, a low radiation dose, and larger reference databases. On top of that, DEXA also measures body composition: how much fat and how much muscle mass you have, and whether vertebral fractures are present.

DEXA also has clear limitations. It produces no three-dimensional image and says nothing about the fine internal structure of bone tissue, the so-called microarchitecture. As a result, fracture risk can be underestimated, particularly in people whose bone structure is already compromised but whose density still looks reasonably normal. Newer techniques such as quantitative CT can provide that information, but are not yet standard practice in the clinic.

In children and adolescents, additional caveats apply: the growing skeleton makes interpretation more complex, and a diagnosis of osteoporosis in this group is never based solely on a DEXA measurement.