How do I interpret the results of a Dual-Energy X-ray Absorptiometry (DEXA) screening?

How to Read a DEXA Screening

Interpret DEXA scans systematically using the PARED approach (Positioning, Artifacts, Regions of Interest, Edge Detection, Demographics), focusing on T-scores for diagnosis in postmenopausal women and men over 50, where T-score ≤ -2.5 indicates osteoporosis, -1.0 to -2.5 indicates osteopenia, and ≥ -1.0 is normal. 1

Step 1: Verify Technical Quality Using PARED

Before interpreting any numbers, systematically evaluate the scan quality 1:

• P – Positioning: Check that the patient's body and limbs are aligned to midline with minimal rotation. Poor positioning invalidates the results 1
• A – Artifacts: Identify surgical hardware, vascular calcifications, undissolved calcium tablets, or contrast material that falsely elevate BMD 1
• R – Regions of Interest: Verify that ROIs are correctly placed on vertebral bodies and femoral neck. On follow-up scans, confirm ROIs match the baseline scan 1
• E – Edge Detection: Ensure bone edges are properly detected by the software 1
• D – Demographics: Confirm correct patient age, sex, ethnicity, and that the appropriate reference database is used 1

Step 2: Identify Which Measurement Sites to Use

Base your diagnosis on the lowest T-score from three standard sites: lumbar spine (L1-L4), femoral neck, or total hip 2:

• Measure BMD at lumbar spine L1-L4, total hip, and femoral neck as standard sites 1
• Exclude vertebrae with T-score differences ≥ 1.0 from adjacent vertebrae, as this indicates structural abnormality 1
• Exclude vertebrae with visible degenerative changes, compression fractures, or surgical hardware 1
• If 2 or more vertebrae are excluded, the lumbar spine site becomes invalid 1

Step 3: Apply WHO Diagnostic Criteria

For postmenopausal women and men over 50, use T-scores 2:

• Normal: T-score ≥ -1.0 2
• Osteopenia (Low Bone Mass): T-score between -1.0 and -2.5 2
• Osteoporosis: T-score ≤ -2.5 2
• Severe Osteoporosis: T-score ≤ -2.5 plus one or more fragility fractures 2

For premenopausal women, men under 50, and children, use Z-scores instead 2:

• Z-score ≤ -2.0 is considered "below expected range for age" 2
• Do not diagnose osteoporosis based on BMD alone in these populations 2

Step 4: Recognize Common Pitfalls That Falsely Elevate BMD

Degenerative spine changes are the most common cause of falsely elevated BMD, leading to underdiagnosis of osteoporosis in elderly patients who need treatment most 3:

• Facet joint osteoarthritis affects 40% of women aged 55 and 85% over age 75, causing spuriously high spine BMD 1
• Osteophytes, vertebral sclerosis, and DISH (diffuse idiopathic skeletal hyperostosis) artificially increase measurements 1
• Aortic calcifications overlying vertebrae falsely elevate BMD 1
• Scoliosis makes DEXA measurements unreliable 1
• One study found 44% of fracture patients were classified as osteoporotic by DEXA versus 81% by quantitative CT, demonstrating significant underdiagnosis 3

When degenerative changes are present, consider alternative imaging: CT Hounsfield units >160 indicate low osteoporosis risk, while <110 correlates with osteoporosis 1, 3

Step 5: Integrate Fracture Risk Assessment

Even with osteopenia (T-score -1.0 to -2.5), treatment is recommended if FRAX shows 10-year hip fracture probability ≥3% or major osteoporotic fracture ≥20% 2:

• Calculate FRAX using age, BMD, and clinical risk factors 1
• An existing osteoporotic fracture supersedes any DEXA measurement and establishes the diagnosis regardless of T-score 2
• Perform Vertebral Fracture Assessment (VFA) during DEXA if T-score <-1.0, historical height loss ≥4 cm, or prospective height loss ≥2 cm 2

Step 6: Determine Follow-Up Timing

Standard follow-up interval is 2-3 years for stable patients, but shorten to 1-2 years for high-risk situations 1, 4:

• 1-year follow-up for patients on glucocorticoids >3 months, starting osteoporosis treatment, or with conditions causing rapid bone loss 4
• 2-year follow-up for patients with risk factors like chronic renal failure, inflammatory arthritis, or medications affecting BMD 4
• Do not repeat scans more frequently than every 6 months, as changes won't be clinically meaningful 1, 4
• Always use the same DXA machine for follow-up and compare absolute BMD values (g/cm²), not T-scores 2, 4

Critical Components Every Report Must Include

The DEXA report must document 1:

• Patient demographics (age, sex, weight, height, menopausal status) 1
• BMD in g/cm² with corresponding T-scores for each site measured 1
• Technical quality statement and any limitations (artifacts, excluded vertebrae) 1
• Comments on degenerative changes or structural abnormalities 1
• FRAX scores if applicable 1
• Diagnosis (normal, osteopenia, or osteoporosis) 1
• Recommendation for follow-up timing 1

Special Populations Requiring Modified Interpretation

For patients on long-term bisphosphonates, add Full Femur Imaging (FFI) to screen for atypical femur fractures 1:

• HIGH likelihood features (focal lateral cortical thickening with transverse lucent line) require urgent consultation 1
• MODERATE likelihood features require dedicated radiographs for clarification 1

For spinal cord injury patients, measure total hip, distal femur, and proximal tibia rather than standard sites 4