Has Your Bone Density Changed?Check the DXA Comparison

A follow-up DXA report can show a small percentage difference without establishing a measurable change in your bones. The ISCD’s 2025 patient resource explains that scan measurements have variability from calibration, positioning and analysis. A facility needs its own threshold to judge whether a difference exceeds that variability.¹

Find the BMD values as well as the T-scores

The updated DXA practice guideline says serial comparisons use bone mineral density, or BMD, measured in grams per square centimetre, written g/cm². T-scores and Z-scores are calculated comparisons with reference populations; they are not the values used to calculate change between scans.²

Keep the full reports, including the measured site. “Total hip” and “femoral neck” identify different regions.² Ask the interpreter to show you which matching regions were compared. Our T-score and Z-score guide explains those reference comparisons.

Ask for the threshold for that site

The ISCD’s official positions recommend that each facility calculate its precision error and LSC, rather than using the manufacturer’s precision estimate. The follow-up report should identify the comparison scan and region, the facility’s LSC and the statistical significance of the result.³

Here is an invented example for a technically valid total-hip comparison:

• Earlier BMD: 0.800 g/cm².
• Follow-up BMD: 0.784 g/cm².
• Difference: 0.016 g/cm² lower, or 2.0% of the earlier value.
• Suppose the applicable LSC is 0.024 g/cm², equivalent to 3.0% of that earlier value.

The measured decrease is smaller than that hypothetical LSC. The updated DXA practice guideline says changes must meet or exceed the LSC to be considered statistically significant.² The example therefore would not establish a significant decrease. It does not prove that absolutely no biological change occurred, and 3% is not a universal threshold.

Ask the clinician to point to your actual facility’s value, in the appropriate units, and interpret your result against it.

Check whether the scanner changed

The ISCD’s official positions recommend returning to the same device when its precision and LSC meet the required standards. A familiar hospital name alone does not tell you whether the scanner is the same.³

If the machine was replaced or you attended another facility, ask whether the systems were cross-calibrated for the site being followed. This is a technical assessment that makes measurements between systems comparable. The ISCD says that without cross-calibration, a numerical comparison with the previous machine cannot be made; a new baseline should be established.³

You can still discuss what the new scan means for current care. Have the report distinguish that assessment from a claim about how much density changed since the earlier scan.

Let the interpreter check the images

The ISCD patient resource also identifies positioning differences, altered analysis regions, new fractures or surgery, degenerative changes and large weight changes as factors that can affect comparison. A spine result can appear improved because of changes such as calcification, rather than stronger bone.¹

Tell the team about changes since your previous scan and ask whether both images were reviewed together. A surprising percentage deserves an image-quality check before you draw a conclusion from the number.

The updated DXA practice guideline also states that an established osteoporosis diagnosis remains relevant even if later T-scores improve above −2.5.² A better number is a reason to review the plan with the clinician, not an instruction to stop treatment yourself.

The “Muscle and Bone” section in My Body Changed. Now What? puts bone-health questions alongside fracture history, medicines and menopause stage. Bring those details and both scan reports to the review, with one question written at the top: “Is this a valid, significant change, and what does it change in my care?”