Understanding a DEXA (DXA) scan

The short version

A DXA scan uses two low-energy x-ray beams to estimate how much mineral is present in a defined area of bone. The machine usually measures the lumbar spine and hip because these sites are important for osteoporosis diagnosis and fracture risk assessment.⁴

The most familiar result is bone mineral density, or BMD. DXA reports also commonly include a T-score, a Z-score, the exact skeletal site measured, and sometimes fracture-risk estimates or follow-up comparisons. A low number matters, but it is not the whole story. Age, prior fracture, medications, medical conditions, fall risk, and scan quality all affect how a clinician uses the result.⁵

Why DXA was developed

Osteoporosis created a practical clinical problem: clinicians needed a way to estimate bone strength before or after a fragility fracture, but ordinary x-rays could miss substantial bone loss and could not reliably quantify small changes. Earlier densitometry methods, including single-photon and dual-photon absorptiometry, helped move bone measurement into quantitative medicine, but they were slower and often depended on radionuclide sources.

DXA emerged in the late 1980s and early 1990s as a faster, more precise way to measure bone mineral and soft-tissue composition. Early DXA systems used an x-ray source to create two energy levels, allowing the computer to separate bone mineral from surrounding soft tissue more effectively.^1,2 Studies soon showed that DXA could measure lumbar spine BMD with good precision and could be used in central skeletal sites that matter clinically.³

The reason this mattered was not just technological. Bone density is strongly related to fracture risk, but fracture risk is not determined by density alone. DXA became useful because it gave clinicians a reproducible starting point: a number that could be combined with age, fracture history, medications, falls, and other clinical risk factors.^8,9

How the test is performed

A typical central DXA scan is done while you lie on a padded table. The scanner arm passes over the body while the x-ray detector measures how much of each energy beam passes through the region being examined. The test is painless and usually takes only a short time.

Before the scan, patients are usually asked to remove metal objects near the scanning area. Prior contrast studies, recent nuclear medicine scans, metal hardware, spine arthritis, compression fractures, or positioning problems can affect interpretation. Pregnancy status should always be discussed with the imaging team before any x-ray-based test.

What the machine is actually measuring

DXA does not directly test whether a bone will break, and it does not measure three-dimensional bone strength. It estimates mineral content from two x-ray energies and divides that mineral content by the projected two-dimensional bone area, producing areal BMD. That is useful, but it also means body size, positioning, artifacts, spine arthritis, compression fractures, hardware, and comparison technique can matter when the report is interpreted.^1,4,5,6

How T-scores are interpreted

The T-score compares a patient’s BMD with a young-adult reference population. For postmenopausal women and men age 50 and older, the World Health Organization densitometric categories are commonly applied to central DXA results.^5,6

• Normal bone density: T-score -1.0 or higher.
• Low bone mass: T-score below -1.0 but above -2.5. The word osteopenia is often used, but low bone mass or low bone density is clearer.
• Osteoporosis by BMD: T-score -2.5 or lower at an accepted diagnostic site, such as the lumbar spine, total hip, femoral neck, or in certain circumstances the one-third radius.
• Clinical context: A fragility fracture can be clinically important even when the T-score is not -2.5 or lower.

The most important idea is that fracture risk rises continuously as BMD falls. There is no magic cliff at -2.5 where bones suddenly become fragile. A T-score helps classify the density result, but treatment decisions usually depend on the whole risk picture.^8,9

How Z-scores are interpreted

The Z-score compares BMD with people of similar age, sex, and ethnicity. It is especially important for premenopausal women, men younger than 50, and children. In these groups, a T-score can be misleading because the clinical question is usually not, How do you compare with a young-adult peak reference? The question is, Is your bone density lower than expected for someone like you?

Current densitometry guidance uses the phrase below the expected range for age when the Z-score is -2.0 or lower. A Z-score above -2.0 is usually described as within the expected range for age. Osteoporosis should not be diagnosed in men younger than 50 on BMD alone, and pediatric reports require special rules that include fracture history and growth context.⁶

How DXA is used in real life

1. To diagnose low bone density or osteoporosis

DXA gives a standardized way to classify bone density at clinically important skeletal sites. The lumbar spine and hip are usually measured in all patients because they are central sites with strong evidence for diagnosis and risk assessment.^4,6

2. To estimate fracture risk

Bone density predicts fracture risk, but it cannot identify every person who will fracture. Meta-analyses show that lower BMD is associated with higher fracture risk, yet many fractures occur in people whose T-scores are not in the osteoporosis range.^8,9 That is why clinicians combine DXA with age, prior fracture, family history, smoking, glucocorticoid exposure, alcohol intake, rheumatoid arthritis, fall risk, and other factors.¹²

3. To support FRAX or another fracture-risk tool

FRAX estimates 10-year probability of hip fracture and major osteoporotic fracture. It can be calculated with or without femoral neck BMD, but femoral neck BMD can sharpen the estimate when available. FRAX is useful because it separates diagnostic labels from absolute risk.¹⁰

4. To monitor change over time

Repeat DXA can help monitor response to therapy or detect ongoing bone loss, but follow-up testing should have a clear purpose. The key question is not simply whether the number changed. The question is whether the change is larger than expected measurement variability and whether it would change care.⁷

Why one number can mislead

DXA is powerful because central DXA is the standard BMD test and BMD predicts fracture risk. It is limited because bone strength is more than mineral density. Bone size, cortical thickness, trabecular architecture, turnover, microdamage, fall mechanics, muscle, vision, medications, and prior fractures all matter. Two people with the same T-score may need different conversations if one has prior fractures, high fall risk, or major medical contributors and the other does not.^8,9

Artifacts can also distort a report. Arthritis, spinal osteophytes, aortic calcification, old compression fractures, surgical hardware, hip replacements, scoliosis, or poor positioning may make a region look denser or less reliable than it really is. A good report should identify the sites measured, any excluded vertebrae, the diagnostic category, fracture-risk information, comparison with prior scans when appropriate, and limitations that affect interpretation.^5,6

Questions to ask after a DXA scan

• Which site had the lowest valid score: lumbar spine, femoral neck, total hip, or one-third radius?
• Was the scan technically good, or were any areas affected by arthritis, fracture, hardware, or positioning?
• Should my result be interpreted with a T-score or a Z-score?
• Does my prior fracture history change what the scan means?
• Was FRAX calculated, and did it include femoral neck BMD?
• If this is a repeat scan, did the BMD change more than the least significant change?
• Do I need lab evaluation for secondary causes of low bone density?
• Should I have spine imaging or vertebral fracture assessment?
• What should I do now, and what should be rechecked later?

A patient-centered way to think about the result

I like to think of a DXA report as a map, not a verdict. It can show where bone reserve is lower than expected, where fracture risk may be rising, and whether bone density is changing over time. But it cannot replace the clinical story. The most useful DXA interpretation connects the numbers to the person: age, fracture history, medical conditions, medications, strength, balance, fall risk, and goals.

If your report is confusing, that does not mean you are missing something obvious. These reports use technical language because they were built for measurement and classification. The goal is to translate the numbers into better questions and a more thoughtful plan with your clinician.

References

1. Watts NB. Fundamentals and pitfalls of bone densitometry using dual-energy x-ray absorptiometry (DXA). Osteoporos Int. 2004;15(11):847-854. doi:10.1007/s00198-004-1681-7.
2. El Maghraoui A, Roux C. DXA scanning in clinical practice. QJM. 2008;101(8):605-617. doi:10.1093/qjmed/hcn022.
3. Licata AA, Binkley N, Petak SM, Camacho PM. Consensus statement by the American Association of Clinical Endocrinologists and American College of Endocrinology on the quality of DXA scans and reports. Endocr Pract. 2018;24(2):220-229. doi:10.4158/CS-2017-0081.
4. Watts NB. Fundamentals and pitfalls of bone densitometry using dual-energy x-ray absorptiometry (DXA). Osteoporos Int. 2004;15(11):847-854. doi:10.1007/s00198-004-1681-7.
5. Morin SN, Leslie WD, Schousboe JT. Osteoporosis: a review. JAMA. 2025. doi:10.1001/jama.2025.6003.
6. Licata AA, Binkley N, Petak SM, Camacho PM. Consensus statement by the American Association of Clinical Endocrinologists and American College of Endocrinology on the quality of DXA scans and reports. Endocr Pract. 2018;24(2):220-229. doi:10.4158/CS-2017-0081.
7. Gourlay ML, Fine JP, Preisser JS, et al. Bone-density testing interval and transition to osteoporosis in older women. N Engl J Med. 2012;366(3):225-233. doi:10.1056/NEJMoa1107142.
8. El Maghraoui A, Roux C. DXA scanning in clinical practice. QJM. 2008;101(8):605-617. doi:10.1093/qjmed/hcn022.
9. Kanis JA, Harvey NC, Johansson H, Liu E, Vandenput L, Lorentzon M, Leslie WD, McCloskey EV. A decade of FRAX: how has it changed the management of osteoporosis? Aging Clin Exp Res. 2020;32(2):187-196. doi:10.1007/s40520-019-01432-y.
10. Kanis JA, Johansson H, Harvey NC, McCloskey EV. A brief history of FRAX. Arch Osteoporos. 2018;13(1):118. doi:10.1007/s11657-018-0510-0.
11. Shevroja E, Lamy O, Kohlmeier L, Koromani F, Rivadeneira F, Hans D. Use of trabecular bone score (TBS) as a complementary approach to dual-energy x-ray absorptiometry (DXA) for fracture risk assessment in clinical practice. J Clin Densitom. 2017;20(3):334-345. doi:10.1016/j.jocd.2017.06.019.
12. Masud T, Binkley N, Boonen S, Hannan MT. Official positions for FRAX clinical regarding falls and frailty: can falls and frailty be used in FRAX? J Clin Densitom. 2011;14(3):194-204. doi:10.1016/j.jocd.2011.05.010.