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GERICLINICS

GeriQBank · Paper III · Q3

DEXA Scan (Dual-Energy X-Ray Absorptiometry) and its Clinical Use

Very High YieldAsked ×37-mark note
Topic
Osteoporosis, Bone & Rheumatology
Chapter
Metabolic Bone Disease & Rheumatology
Years asked
Not recorded in the source
Frequency
Set 3× in Paper III (2009–2025)

As set in the university paper

  • Dual energy X-ray absorptiometry — DEXA scan and its clinical uses; and how the bone mineral density test is interpreted, with a note on the FRAX tool.

    Paper III · Bone, Joint & Rheumatology · asked ×3 · 7-mark note · see in paper list

Answer framework

  1. Principle — two X-ray energies; areal bone mineral density; the reference standard
  2. Sites — spine, hip, distal radius
  3. Interpretation — T-score and Z-score cut-offs
  4. Safety and precision
  5. Clinical uses — diagnosis, FRAX, intervention thresholds, monitoring, vertebral fracture assessment, trabecular bone score, body composition
  6. Who to scan
  7. Pitfalls in older people and monitoring rules
  8. Recent updates — ISCD 2023, goal-directed treatment, Indian reference data

An exam answer plan drawn from the detailed answer below. Use it to attempt the question first.

Detailed answer

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Attempt it firstOutline your answer as a 7-mark note before revealing it — definition, causes, evaluation, management and the geriatric angle.Reveal answer

Short revision answer

  • DXA passes two X-ray beams of different energies through the body; differential attenuation gives areal bone mineral density (g/cm²). It is the reference standard.
  • Sites: lumbar spine (L1–L4), femoral neck and total hip; the distal one-third radius when the spine or hip cannot be used, in hyperparathyroidism and in very obese patients.
  • T-score: ≥−1.0 normal; −1.0 to −2.5 osteopenia; ≤−2.5 osteoporosis; with a fragility fracture, severe osteoporosis. Use the Z-score in men under 50 and premenopausal women; a Z-score ≤−2.0 needs a work-up for secondary causes.
  • The radiation dose is very low, the scan is quick, and it is precise enough for serial monitoring.
  • Uses: diagnosis, FRAX input, treatment thresholds, monitoring every 1–3 years, vertebral fracture assessment, trabecular bone score, and lean mass for sarcopenia.
  • Scan all women from 50 and all men by 65 (Pathy's), and anyone with a fragility fracture, height loss, glucocorticoid treatment or a secondary cause.
  • In older people, spinal BMD is falsely raised by osteophytes, compression fractures, aortic calcification, scoliosis, hardware and contrast, so the hip is more reliable. Exclude a vertebra that differs from its neighbours by more than 1.0, use the lowest valid site, and repeat scans on the same machine.
  • Updates: ISCD 2023 supports reporting trabecular bone score and opportunistic CT-based screening; ASBMR/BHOF 2024 treats to a target T-score above −2.5; use Indian reference data, as Caucasian norms over-diagnose osteoporosis in Indian patients.

Condensed by GeriClinics from the detailed answer below, with no new facts added.

1.

Principle: two X-ray beams of different energies are passed through the body; differential attenuation by bone and soft tissue allows calculation of bone mineral content and areal bone mineral density (g/cm²). It is the reference standard for measuring BMD.

2.

Standard sites: lumbar spine (L1–L4) and hip (femoral neck and total hip); the distal one-third radius is used when spine or hip cannot be measured, or in hyperparathyroidism and very obese patients.

3.

T-score compares BMD with the young adult mean: ≥−1.0 normal, −1.0 to −2.5 osteopenia, ≤−2.5 osteoporosis, and ≤−2.5 with a fragility fracture = severe (established) osteoporosis. Z-score compares with age-matched peers and is used in men under 50 and premenopausal women; Z ≤ −2.0 means "below expected for age" and mandates a secondary-cause work-up.

4.

Radiation dose is very low (a few microsieverts, less than a chest radiograph), the scan takes minutes, and it is precise enough for serial monitoring.

5.

Clinical uses: (a) diagnosis of osteoporosis; (b) fracture-risk prediction and input into FRAX; (c) deciding intervention thresholds; (d) monitoring response to therapy, repeated every 1–3 years; (e) Vertebral Fracture Assessment (VFA) from the same machine; (f) Trabecular Bone Score (TBS) as a texture measure of bone quality; (g) body-composition and appendicular lean mass measurement for the diagnosis of sarcopenia.

6.

Who to scan: Pathy's advises BMD in all women from 50 years and all men by 65 years, repeating in two years (in the same season) if results are borderline; also anyone with a fragility fracture, height loss, glucocorticoid therapy, or a secondary cause.

7.

Pitfalls that falsely raise spine BMD in older people: degenerative osteophytes and facet arthrosis, vertebral fracture, aortic calcification, scoliosis, previous instrumentation, and residual contrast — hence in the elderly the hip is the more reliable site. Exclude any vertebra whose T-score differs from adjacent ones by more than 1.0; at least two vertebrae must be evaluable. Diagnosis uses the lowest of the valid sites. Repeat scans should be on the same machine, since inter-machine comparison is invalid without cross-calibration, and change is meaningful only if it exceeds the least significant change.

Latest Updates (2023 – Present) — 3 Points

ISCD 2023 Official Positions endorse routine reporting of TBS alongside BMD and permit opportunistic CT-based BMD screening when a CT has been done for another reason.

ASBMR/BHOF 2024 goal-directed treatment converts DXA from a diagnostic test into a treatment target: the aim is a T-score above −2.5, and the drug is chosen by the likelihood of reaching that goal. Indian reference data: ISBMR advises Indian young-adult reference databases and cautions that Caucasian NHANES norms over-diagnose osteoporosis in Indian patients; DXA-based appendicular lean mass indices are also used with AWGS 2019 Asian cut-offs for sarcopenia.

Extra Recall Scaffold — Recurring Topic

Mnemonic for falsely high spine BMD — "SCAFOLD": Scoliosis · Compression fracture · Aortic calcification · Facet joint / osteophyte degeneration · Osteomalacia treatment effect · Laminectomy hardware / instrumentation · Dye (residual contrast).

Numerical anchors: normal ≥−1.0 · osteopenia −1 to −2.5 · osteoporosis ≤−2.5 · Z-score ≤−2.0 abnormal · monitor every 1–3 years · adjacent-vertebra T-score difference >1.0 = exclude · women from 50, men by 65. Examiner traps: (1) Using T-score in a premenopausal woman or young man — use Z-score. (2) Reporting the average of sites rather than the lowest. (3) Forgetting that severe spinal degeneration in the elderly makes spine BMD unreliable. (4) Comparing scans from two different machines. (5) Saying DXA measures volumetric density — it measures areal density, which underestimates BMD in small-framed people (relevant to Indian patients).

15-second answer skeleton: Principle → sites → T-score and Z-score with cut-offs → safety and precision → seven clinical uses → who to scan → pitfalls in the elderly and monitoring rules.

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