A single DEXA scan tells you what your bones look like today. To know whether they are getting better, holding steady, or continuing to lose density, you need to compare scans over time. But comparing DEXA scans is not as simple as "your number went down — that's bad." Real bone-density change has to be separated from the normal scan-to-scan measurement noise.
This guide explains how to read serial DEXA scans, when to repeat them, and what the term "least significant change" means in your report.
Why repeat a DEXA scan?
Repeat DEXA scans are done to:
- Confirm a diagnosis — sometimes a borderline result is rechecked after 1 to 2 years to be sure.
- Monitor untreated low bone density — to see whether bone loss is progressing or stable.
- Assess response to treatment — to confirm that an osteoporosis medication is working.
- Restart the conversation — after stopping a medication, after menopause, or after starting a drug that affects bone.
How often should DEXA be repeated?
This is one of the most common patient questions, and the honest answer is: it depends on why the first scan was done. Typical intervals:
- Normal first scan, average risk → repeat in 5 to 10 years (or earlier if risk factors change).
- Osteopenia, no treatment → repeat in 2 to 5 years.
- Osteoporosis, just started treatment → repeat in 1 to 2 years to assess response.
- On stable treatment with a good response → repeat every 2 to 3 years.
- After stopping treatment (drug holiday) → repeat in 1 to 2 years.
Repeating sooner than 1 year is rarely useful, because real bone-density change is slow and any apparent difference is more likely to be measurement noise.
What is the "least significant change" (LSC)?
Every DEXA machine has a small amount of measurement variability — the same patient scanned twice on the same day will get slightly different numbers. The Least Significant Change is the smallest difference between two scans that is bigger than this noise and therefore likely to be real.
- LSC is usually expressed as a percentage and is typically around 3 to 5 percent for the spine and hip on modern DEXA machines.
- An LSC value should be calculated for each individual scanner — it depends on the machine, the radiographer, and the centre's quality control.
- A change in your BMD that is smaller than the LSC is considered "no significant change."
- A change larger than the LSC in the same direction is considered real.
If your second scan's BMD is 1.5% different from your first scan but the centre's LSC is 4%, your bones have not measurably changed. This is normal and not bad news.
What about T-score changes between scans?
Doctors usually monitor changes in raw bone mineral density (g/cm²), not the T-score, because:
- T-scores are based on reference databases that vary slightly between machines.
- Small changes in raw BMD can produce confusing-looking T-score changes that are not actually meaningful.
You may see the T-score change between scans, but the doctor's interpretation is usually anchored on the BMD trend at each site.
Why repeat on the same machine?
Different DEXA machines are not directly comparable
Different DEXA models calibrate slightly differently. A 5% drop on one machine may not equal a 5% drop on another. When you go for a follow-up DEXA, try to use the same imaging centre, ideally the same scanner, as your previous study. Bringing your previous report and images is essential — without them the radiologist cannot quantify change reliably.
If you have to switch centres, the new centre may need to do a "cross-calibration" or simply treat the new scan as a fresh baseline.
What does a good response to treatment look like?
For most osteoporosis medications:
- A stable or improving BMD after 1 to 2 years is considered a good response. "No change" on treatment is often the goal — it means bone loss has been halted.
- A drop in BMD that exceeds the LSC suggests the treatment is not working, the patient is not taking it consistently, or there is a secondary cause that has not been addressed.
Spine BMD often responds first and largest to treatment, because of its higher trabecular content. Hip BMD changes more slowly. Both sites are worth tracking.
Can my DEXA result get worse even with treatment?
Yes, but it is uncommon if the treatment is being taken correctly. The most common reasons for an apparent "worsening" while on treatment are:
- Not taking the medication consistently — particularly oral bisphosphonates, which require specific dosing on an empty stomach.
- Vitamin D deficiency — treatment works much less well when vitamin D levels are low.
- An undiagnosed secondary cause — such as hyperparathyroidism, malabsorption, or excessive thyroid replacement.
- Measurement variability below the LSC that is misread as "worsening."
A genuine drop in BMD on treatment is worth investigating, not just shrugging off.
What if I have not had a fracture but my numbers got worse?
A real (above-LSC) decline in BMD without a fracture is still important information. It usually triggers a discussion about whether to escalate treatment, look for a secondary cause, or address adherence. It is rare for someone to have meaningful bone loss while taking osteoporosis medication correctly.
Vertebral fracture assessment on the same visit
Some centres now include a vertebral fracture assessment (VFA) — a low-dose DEXA image of the spine — at follow-up scans. This is useful because vertebral fractures often occur silently and may change the treatment plan even when the BMD looks stable.
Conclusion
A repeat DEXA is most valuable when there is a specific question to answer — confirming treatment response, checking on progressive bone loss, or revisiting a borderline result. The change between scans should be measured against the centre's least significant change before drawing conclusions. Use the same imaging centre when you can, bring your previous report, and read the trend over years rather than reacting to a single number.