The Gold Standard for Bone Density Measurement
Normal: T-score at or above -1.0 SD
Osteopaenia (low bone mass): T-score between -1.0 and -2.5 SD
Osteoporosis: T-score at or below -2.5 SD
Severe (established) Osteoporosis: T-score at or below -2.5 SD WITH one or more fragility fractures
Key: The T-score threshold of -2.5 was derived from population data showing fracture risk increases exponentially below this level
- DXA measures areal bone mineral density (aBMD) in g/cm² using two X-ray beams of different energies to separate bone from soft tissue.
- T-score compares the patient's BMD to the mean of a young adult reference population. Each 1 SD decrease doubles fracture risk.
- WHO classification: Normal = T-score at or above -1.0, Osteopaenia = T-score between -1.0 and -2.5, Osteoporosis = T-score at or below -2.5.
- FRAX integrates BMD with clinical risk factors to estimate 10-year probability of major osteoporotic and hip fractures.
- Z-score (not T-score) is used in premenopausal women, men under 50, and children — compares to age-matched controls.
- “DXA measures AREAL density (g/cm²), not TRUE volumetric density (g/cm³) — this means it overestimates BMD in large bones and underestimates in small bones.
- “Spine DXA is falsely elevated by: degenerative osteophytes, compression fractures, aortic calcification, scoliosis — always check the image.
- “For monitoring treatment response, the Least Significant Change (LSC) must be exceeded — typically 3-5% for spine, 4-6% for hip.
- “The hip (femoral neck specifically) is the best predictor of hip fracture — it should always be included in DXA scanning.
- “Vertebral fracture assessment (VFA) can be performed simultaneously with DXA and identifies prevalent fractures that change management.
DXA and osteoporosis management are commonly examined in both clinical and viva settings. You must be able to: explain the physics of DXA, interpret T-scores and Z-scores, discuss sources of error that falsely elevate or lower BMD, explain the FRAX algorithm, and outline internationally accepted treatment thresholds. A common viva trap is using T-scores in premenopausal women or men under 50 (where Z-scores should be used).
NO-OSWHO Classification
Hook:NO-OS: Normal (above -1), Osteopaenia (-1 to -2.5), Osteoporosis (below -2.5), Severe (below -2.5 + fracture).
DOCSSources of Falsely Elevated DXA
Hook:DOCS: when spine DXA looks surprisingly good, check for these four common causes of false elevation.
FRAX-10FRAX Clinical Risk Factors
Hook:FRAX: Fracture history, RA/risks, Alcohol/smoking, eXcessive steroids — the key clinical inputs to the 10-year fracture risk calculator.
Overview
Dual-energy X-ray Absorptiometry (DXA or DEXA) is the gold standard for measuring bone mineral density (BMD) and diagnosing osteoporosis. It is the most widely validated, precise, and clinically applicable bone density measurement technique, forming the basis of the WHO diagnostic classification and treatment guidelines worldwide.
DXA uses two X-ray beams of different energies (typically 40 and 70 keV) to separately measure the attenuation by bone mineral and soft tissue. By mathematically combining the two measurements, the contribution of soft tissue is eliminated, leaving a measurement of bone mineral content that is divided by the projected bone area to give areal bone mineral density (aBMD) in g/cm².
BMD is the single strongest predictor of fracture risk that can be measured clinically. For each 1 standard deviation decrease in BMD, fracture risk approximately doubles. However, BMD alone does not capture all fracture risk — clinical risk factors (age, prior fracture, glucocorticoids, family history) modify fracture probability independently of BMD. This is why FRAX was developed: to integrate BMD with clinical risk factors into a single 10-year fracture probability estimate.
DXA measures areal BMD (g/cm²) — bone mineral content divided by the projected two-dimensional area. This means DXA inherently measures a combination of bone density and bone size. Large bones will have higher areal BMD than small bones even if their true volumetric density (g/cm³) is identical. This is clinically relevant: DXA may overestimate BMD in tall individuals with large vertebrae and underestimate it in short individuals with small vertebrae. Quantitative CT (QCT) can measure true volumetric BMD but is not used for routine clinical diagnosis.
Clinical Imaging
Imaging Atlas
Systematic Approach
Systematic DXA Interpretation
| Step | Assessment | Key Considerations |
|---|---|---|
| 1. Review the image | Check the DXA scan image for artefacts and positioning errors | Exclude vertebrae with compression fractures, osteophytes, or overlying calcification from the analysis |
| 2. Identify the correct score | T-score for postmenopausal women and men over 50. Z-score for premenopausal women, men under 50, and children | Using the wrong score is a common error — T-scores are meaningless in young patients |
| 3. Apply WHO classification | Normal (above -1.0), Osteopaenia (-1.0 to -2.5), Osteoporosis (at or below -2.5) | The lowest T-score at any measured site determines the overall diagnosis |
| 4. Calculate FRAX if indicated | Enter BMD and clinical risk factors into the FRAX calculator | FRAX gives 10-year probability of major osteoporotic fracture and hip fracture separately |
| 5. Apply treatment thresholds | Treat if T-score at or below -2.5, or if a fragility fracture is present regardless of T-score. In osteopaenia, use FRAX-based intervention thresholds (e.g. NOGG age-dependent threshold, or the US fixed threshold of 10-year hip risk at or above 3% / major osteoporotic fracture at or above 20%) | Thresholds differ by country and FRAX calibration — quote the principle, not a single national cut-off |
| 6. Plan monitoring | Repeat DXA at 1-2 years to assess treatment response or disease progression | Change must exceed the Least Significant Change (LSC) to be clinically meaningful — typically 3-5% change |
DXA Physics and Technique
Dual-Energy X-ray Absorptiometry Physics
DXA exploits the principle that bone mineral and soft tissue have different X-ray absorption characteristics at different energies. By measuring the attenuation of two X-ray beams of different energies (typically approximately 40 keV and approximately 70 keV), the system mathematically separates the contribution of bone from soft tissue.
How it works:
- An X-ray tube generates a broadband X-ray beam
- The beam is filtered to produce two distinct energy peaks (K-edge filtration with cerium or samarium filters, or rapid kV switching)
- Each energy beam is attenuated differently by bone mineral (calcium hydroxyapatite) and soft tissue
- Detectors measure the transmitted intensity at both energies
- Mathematical algorithms separate bone mineral content (BMC in grams) from soft tissue
- BMC is divided by the projected bone area to give areal BMD (g/cm²)
Measurement sites:
- Lumbar spine (L1-L4): Anteroposterior projection. Most responsive to treatment changes but susceptible to artefact from degenerative disease.
- Proximal femur: Total hip and femoral neck are measured. Femoral neck is the most important predictor of hip fracture. Less susceptible to degenerative artefact than the spine.
- Distal radius (1/3 radius): Used when the spine and hip cannot be measured (e.g., bilateral hip replacements, severe spinal degeneration).
Radiation dose: DXA produces an extremely low radiation dose — approximately 0.001 mSv per scan (less than one day of background radiation). This makes it safe for serial monitoring.
FRAX and Clinical Decision-Making
FRAX Fracture Risk Assessment Tool
FRAX is a computer-based algorithm developed by the WHO Collaborating Centre that calculates the 10-year probability of hip fracture and major osteoporotic fracture (hip, spine, forearm, or proximal humerus) based on individual patient risk factors, with or without BMD.
| Variable | Description | Impact on Risk |
|---|---|---|
| Age | Patient age in years (valid for ages 40-90) | Fracture risk increases exponentially with age independent of BMD |
| Sex | Male or female | Women have higher absolute fracture risk at any given T-score |
| Body mass index | Weight in kg / height in m² | Both very low BMI (less than 20) and very high BMI modify risk |
| Prior fragility fracture | Any previous osteoporotic fracture | One of the strongest risk factors — approximately doubles 10-year risk |
| Parental hip fracture | Mother or father had a hip fracture | Strong genetic risk factor independent of BMD |
| Current smoking | Active smoker at the time of assessment | Independent risk factor for fracture (also reduces BMD) |
| Glucocorticoids | Current or recent use (prednisolone 5mg or more daily for 3+ months) | Major secondary osteoporosis risk factor |
| Rheumatoid arthritis | Confirmed diagnosis of RA | Independent risk factor beyond its effect on BMD |
| Secondary osteoporosis | Type 1 diabetes, osteogenesis imperfecta, untreated hyperthyroidism, hypogonadism | Increases risk independent of BMD |
| Alcohol (3+ units/day) | Excessive alcohol intake | Dose-dependent increase in fracture risk |
| Femoral neck BMD (optional) | Can calculate with or without BMD | Adding BMD significantly improves prediction accuracy |
Across guidelines, pharmacological treatment is indicated when: (1) T-score at or below -2.5 at the spine, total hip, or femoral neck. (2) A minimal-trauma (fragility) fracture has occurred — particularly hip or vertebral fracture — regardless of T-score. (3) FRAX 10-year fracture probability crosses an intervention threshold in patients with osteopaenia. Threshold approaches differ: the UK NOGG uses an age-dependent threshold set at the risk equivalent to a woman with a prior fracture, whereas US National Osteoporosis Foundation guidance uses a fixed threshold (10-year hip fracture probability at or above 3% or major osteoporotic fracture at or above 20%). (4) Glucocorticoid-induced osteoporosis warrants treatment at higher BMD (e.g. T-score below -1.5) because bone loss is rapid and predominantly trabecular.
Differential of Low Bone Density and Comparison of Modalities
Modalities for Bone Status Assessment
DXA is the diagnostic standard, but the examiner may ask how it compares with alternative techniques and how to distinguish osteoporosis from other causes of radiographically "lucent" or biochemically abnormal bone.
| Modality | What it measures | Strengths | Limitations / Role |
|---|---|---|---|
| DXA (central, spine and hip) | Areal BMD (g/cm²); T-score and Z-score | Standard for diagnosis, fracture prediction, treatment monitoring; very low dose | 2D areal measure, confounded by degenerative artefact; cannot separate cortical from trabecular bone |
| Quantitative CT (QCT) | True volumetric BMD (g/cm³); separates trabecular and cortical compartments | Not affected by osteophytes or aortic calcification; sensitive to early trabecular loss | Higher radiation dose; not used for WHO T-score diagnosis; less standardised |
| Trabecular Bone Score (TBS) | Grey-level texture index of bone microarchitecture from the spine DXA image | BMD-independent fracture predictor; can adjust FRAX probability | Software add-on; affected by soft-tissue thickness; adjunct not a diagnostic test |
| Quantitative ultrasound (QUS) | Calcaneal speed of sound / broadband attenuation | Radiation-free, portable, low cost — useful in limited-resource and screening settings | Cannot diagnose osteoporosis by WHO criteria; used for risk stratification only |
| Plain radiography | Cortical thinning, lucency, fracture | Detects fragility fractures and gross structural change | Insensitive — 30 to 50% of bone mass must be lost before lucency is visible; cannot quantify BMD |
| Diagnosis | Distinguishing Features | Key Tests |
|---|---|---|
| Primary osteoporosis | Postmenopausal or age-related; normal biochemistry; reduced BMD | DXA, normal calcium/phosphate/ALP, vitamin D |
| Osteomalacia | Defective mineralisation; bone pain, Looser zones; low vitamin D | Low calcium/phosphate, high ALP, low 25-OH vitamin D, high PTH |
| Primary hyperparathyroidism | Cortical (forearm) bone loss predominates; renal stones | High calcium, high PTH; distal radius DXA disproportionately low |
| Multiple myeloma / malignancy | Lytic lesions, anaemia, renal impairment, raised ESR | Serum/urine electrophoresis, marrow biopsy, skeletal survey or MRI |
| Renal osteodystrophy | Chronic kidney disease; mixed osteomalacia and high turnover | Calcium, phosphate, PTH, vitamin D, eGFR |
| Secondary osteoporosis | Glucocorticoids, hypogonadism, thyrotoxicosis, coeliac disease | Targeted screen (TFTs, testosterone/oestradiol, coeliac serology, cortisol) |
Controversies and Areas of Uncertainty
The T-score depends entirely on which young-adult reference population is used. ISCD recommends a uniform Caucasian female NHANES III femoral neck reference for all (so that a man is compared with young women at the hip), but spine and ethnicity-specific references vary between manufacturers and countries. The same scan can yield different T-scores on different databases — a reminder that the diagnostic threshold is a statistical construct, not a biological constant.
Most fragility fractures occur in people with osteopaenic rather than osteoporotic T-scores, because they are far more numerous. This is the central rationale for FRAX: targeting absolute fracture risk rather than a BMD number. The debate is whether to use a fixed intervention threshold (simple, US approach) or an age-dependent threshold (UK NOGG), and how to handle the new "very high risk" category that favours anabolic-first sequencing.
The FLEX and HORIZON extension data support reassessing bisphosphonate-treated patients after 3-5 years and considering a holiday in those at lower risk, while continuing in high-risk patients. There is no equivalent holiday for denosumab — abrupt cessation causes rebound bone loss and multiple vertebral fractures, so transition to a bisphosphonate is required. DXA is central to these decisions but the optimal monitoring interval is debated.
DXA cannot capture bone quality (microarchitecture, turnover, geometry). TBS, hip structural analysis, QCT/finite-element analysis and high-resolution peripheral QCT add information, but none has displaced central DXA for routine diagnosis. Whether these refine treatment decisions enough to justify routine use remains an active question.
Evidence Base
BMD and Fracture Risk Prediction
- Pooled 11 prospective cohorts (~90,000 person-years, over 2000 fractures); each 1 SD fall in BMD raised fracture relative risk by about 1.5 (95% CI 1.4-1.6) at most sites.
- Site-specific measurement was superior for matched outcomes — hip BMD best predicted hip fracture (RR 2.6, 95% CI 2.0-3.5).
- Spine BMD best predicted vertebral fracture (RR 2.3, 95% CI 1.9-2.8).
FRAX Development — Clinical Risk Factors and BMD
- FRAX integrates clinical risk factors (BMI, prior fracture, parental hip fracture, glucocorticoids, rheumatoid arthritis, secondary osteoporosis, smoking, alcohol 3+ units/day) drawn from prior international meta-analyses, with or without femoral neck BMD.
- Four models compute 10-year probability of hip fracture and of major osteoporotic fracture, with and without BMD, calibrated to UK fracture and death epidemiology.
- Across age, 10-year hip fracture probability in women ranged from 0.2% (age 50, no risk factors) to about 22% (age 80 with parental hip fracture) — an approximately 100-fold range — with risk factors adding incrementally.
DXA and FRAX are well-validated for fracture risk prediction and treatment guidance.
Guidelines, Registries & Global Practice
Global Epidemiology
Osteoporosis is a global public-health problem. Worldwide, roughly 1 in 3 women and 1 in 5 men over the age of 50 will sustain an osteoporotic fracture in their remaining lifetime. Hip fractures are the most disabling: incidence rises exponentially with age and varies several-fold between regions, being highest in Northern Europe and lowest in parts of Africa and Asia, with the absolute global burden shifting towards Asia as populations age. DXA access remains uneven — scanners are concentrated in high-income settings, so FRAX without BMD and quantitative ultrasound retain an important triage role where central DXA is scarce.
Side-by-Side Guidance
| Body / Region | DXA indication & diagnosis | Intervention threshold |
|---|---|---|
| WHO / IOF | T-score at or below -2.5 at femoral neck (reference: NHANES III young white women) defines osteoporosis; FRAX for risk | Country-specific FRAX calibration recommended |
| NOGG / NICE (UK) | DXA in those above the FRAX assessment threshold or with clinical risk factors | Age-dependent FRAX threshold set at the risk of a woman with a prior fragility fracture |
| NOF / Endocrine Society (US) | DXA for all women 65+ and men 70+, and younger adults with risk factors | Fixed FRAX threshold: 10-year hip risk at or above 3% or major osteoporotic fracture at or above 20% |
| ISCD (international) | Defines acquisition, ROI selection, LSC and reporting standards; Z-score for young patients | Sets technical standards rather than drug thresholds |
| IOF / ESCEO (Europe) | DXA plus VFA and FRAX; TBS as an adjunct near thresholds | FRAX-based, with very-high-risk category favouring anabolic-first sequencing |
Registries and Audit
National hip-fracture registries and audits (for example the UK National Hip Fracture Database, and equivalents in several European, Asian and Australasian systems) and Fracture Liaison Service registries track secondary-fracture prevention. They consistently show a large treatment gap: most patients who sustain a fragility fracture never undergo DXA or receive bone-protective therapy — a recurring exam point about systems of care.
High- vs Limited-Resource Practice
In well-resourced systems, central DXA with VFA (and increasingly TBS) is standard, with FRAX calibrated to national fracture data. In limited-resource settings, central DXA may be unavailable; here FRAX without BMD, calcaneal quantitative ultrasound, and clinical case-finding (prior fragility fracture, glucocorticoid use, low body weight) guide treatment. The principle is unchanged worldwide: a low-trauma hip or vertebral fracture warrants bone-protective treatment irrespective of whether a T-score is obtainable.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“You receive a DXA report for a 68-year-old postmenopausal woman showing a lumbar spine T-score of -1.8 and a femoral neck T-score of -2.7.”
“A 35-year-old premenopausal woman on long-term oral prednisolone for rheumatoid arthritis has a DXA showing a lumbar spine T-score of -2.8.”
“An examiner asks you to explain why the DXA-measured T-score at the lumbar spine may be falsely normal or elevated in an elderly patient with multiple clinical risk factors for osteoporosis.”
WHO Classification (T-scores)
- Normal: T-score at or above -1.0
- Osteopaenia: T-score -1.0 to -2.5
- Osteoporosis: T-score at or below -2.5
- Severe: T-score at or below -2.5 PLUS fragility fracture
When to Use Z-score
- Premenopausal women, men under 50, and children
- Z-score at or below -2.0 = 'below expected range for age'
- Z-score above -2.0 = 'within expected range'
- T-scores are NOT VALID in these populations
Sources of False Elevation (DOCS)
- Degenerative osteophytes (10-15% overestimation)
- Overlying calcification (aortic calcification)
- Compression fractures (same mineral in smaller area)
- Scoliosis and surgical hardware
FRAX
- 10-year probability of major osteoporotic fracture and hip fracture
- Inputs: age, sex, BMI, prior fracture, parental hip fracture, smoking, steroids, RA, alcohol
- US fixed threshold: hip risk at or above 3% or major fracture at or above 20%; UK NOGG uses an age-dependent threshold
- Always use the FRAX model calibrated to the patient's own country
Monitoring
- Repeat DXA every 1-2 years on the SAME machine
- Change must exceed LSC (typically 3-5% spine, 4-6% hip) to be meaningful
- Spine is most responsive to treatment changes
- Use hip as primary site in elderly patients with degenerative spine