The Clear Picture · Practical Guides

“Why is my DXA spine score so much lower than my hip score?”

If you have had a DXA bone density scan, you may have noticed that the number for your spine and the number for your hip are not the same — sometimes very different. This is extremely common, and it usually has a sensible explanation. This guide explains, in plain terms, why the two sites can disagree, and what a low spine score does and does not mean.

What a DXA scan actually measures

A DXA scanner passes a very low dose of X-rays through your body and works out how much bone mineral is in the area it looks at. To do this, it has to assume that everything in the X-ray path which is not bone is one single, uniform kind of soft tissue.

In real life that is never quite true. Along the path of the beam there is fat, muscle, intra-abdominal organs, blood vessels and blood, as well as the marrow inside the bone. These tissues absorb X-rays by different amounts, and the scanner cannot tell them apart. So the number it gives you is a very useful estimate — but an estimate that can be nudged up or down by things that have nothing to do with how strong your bones really are.

Why the spine and hip so often disagree

Disagreement between the hip and spine is the rule, not the exception. In one large study, the two sites differed by more than 10% in over nine out of ten people. There are several reasons, and in many people more than one is acting at once.

1. The spine and hip are built differently. The spine bones are made almost entirely of the soft, sponge-like inner trabecular bone (with a lot of marrow), while the hip has a shell of dense outer cortical bone around it. Because of the way DXA works, the spongy, marrow-rich spine is more easily thrown off by the soft tissue around and inside it than the hip is. So even when nothing is wrong, the spine number tends to be the more “nervous” of the two.
2. Body shape and the amount of fat. The scanner is sensitive to how much fat sits in the area being measured. Because people carry fat differently in the tummy versus the hips and thighs, the amount of fat over your spine can differ from the amount over your hip — and this alone can make the two scores move in different directions. In people who are slim, there is less fat in the picture, which tends to make the bone look less dense than it truly is, especially at the spine.
3. The size of your bones. This is a big and often overlooked one. DXA does not measure bone in three dimensions — it measures a flat “shadow” of the bone and divides by the area of that shadow. This means a smaller bone tends to score lower, and a larger bone tends to score higher, even if the bone material itself is equally dense. Laboratory experiments using artificial spine bones of different sizes (but identical, known density) have confirmed this directly: the larger models read as denser and the smaller models as less dense, purely because of size. If your spine bones are on the small side, which often goes with a slim, smaller frame, your spine score can look low for reasons of geometry, not weakness.
4. Technical and positioning factors. How you were positioned, whether the bones were labelled correctly, arthritis, old fractures, calcium in nearby blood vessels, surgical metalwork and similar factors can all push a score up or down. Most of these tend to push the spine score up (making it look better than it is); only a few push it down. This is one reason a genuinely low spine score is worth a careful technical look.

A worked example: a hip T-score of −2.7 and a spine T-score of −4.5 in a slim, fit, healthy woman with no fracture history

Here the spine is reading much lower than the hip, in someone who is fit, has never broken a bone, and has a slim build (a modestly low body weight for height). The striking thing is that several of the explanations above all push in the same direction — they all tend to lower the spine score — which is exactly why the gap is so wide.

Put together, these effects can produce a spine score well below the hip score in a slim, fit person without any disease being present. The fact that you are well and have never had a fracture is reassuring, and fits with at least part of that very low spine number being a measurement effect rather than fragile bone.

The bigger point to hold on to

A bone density score is a helpful guide, but it is not the same as bone strength, and it is by no means a perfect predictor of whether you will break a bone. Many people with low scores never fracture, and quite a lot of people with reasonable scores do. Your overall picture — your fitness and level of exercise, your history of fractures, your age, your family history, your diet and other risk factors — matters at least as much as a single number on a scan.

The best way to read your result is this: the hip and spine disagree for understandable reasons; much of the very low spine figure is likely to be a measurement effect in a slim person with smaller bones; and the next step is a calm technical review of the scan with the clinician who reported it. After that, a scan using a different technique such as REMS — which is much less affected by body composition — can help. Alternatively, if you have had a CT or MRI scan of the spine for any reason, bone density can be estimated from those scans as a further guide to the true value.

In summary, a highly discordant set of DXA results with a very low spine score of −4.5 is not a reason to be frightened — but it is a reason to find out more and make proper sense of the result.

This Practical Guide, produced by the Education Directorate of IIMHE, is only for educational and communication purposes. It gives general information to help you understand your DXA results. It is not a diagnosis or personal medical advice. Please discuss your own scan and results with the clinician who arranged or reported them. · IIMHE, July 2026