ISCD CCD cheat sheet

T-score = (patient BMD − young adult mean BMD) / young adult SD; Z-score = (patient BMD − age-matched mean BMD) / age-matched SD.

WHO diagnostic thresholds by T-score at spine, hip, or 33% radius: ≥ −1.0 normal; −1.0 to −2.5 osteopenia (low bone mass); ≤ −2.5 osteoporosis; ≤ −2.5 plus fragility fracture = severe (established) osteoporosis.

Z-score, not T-score, is used to diagnose low bone mass in premenopausal women, men under 50, and children; Z ≤ −2.0 is "below the expected range for age."

Diagnostic classification uses the lowest T-score among valid measured sites; a single site cannot be used if it is technically invalid (e.g., artifact, prior fracture, prosthesis).

DXA precision is expressed as CV (%) and least significant change (LSC); LSC = 1.96 × √2 × precision error, so a change must exceed LSC to be considered real at 95% confidence.

In vivo precision target for lumbar spine BMD is typically ≤ 1.0% CV; hip precision is generally ≤ 1.5–2.0% CV.

BMD is areal (g/cm²) on DXA, so it is influenced by bone size; larger bones read higher and smaller bones lower than true volumetric density.

Exclude vertebrae from spine analysis if they are fractured, have focal structural change, are instrumented, or show artifact; report at least two evaluable vertebrae.

Effective dose from a DXA scan is very low (roughly 1–10 µSv, comparable to a few hours of background radiation).

Indications for BMD testing include women ≥ 65 and men ≥ 70, postmenopausal women and men 50–69 with clinical risk factors, and adults with fragility fracture.

FRAX estimates 10-year probability of major osteoporotic fracture and hip fracture; it is validated for untreated patients and does not apply to those already on therapy.

Treat pharmacologically at FRAX 10-year major osteoporotic probability ≥ 20% or hip fracture probability ≥ 3% (NOF/US thresholds).

Secondary causes to screen before or alongside DXA: hyperparathyroidism, vitamin D deficiency, hyperthyroidism, celiac disease, multiple myeloma, hypogonadism, and glucocorticoid exposure.

Glucocorticoid-induced osteoporosis: bone loss is fastest in the first 3–6 months; BMD monitoring is recommended within 6 months of starting and then annually.

Vertebral fracture assessment (VFA) on DXA detects moderate/severe vertebral deformities and adds risk information independent of BMD.

Repeat DXA is generally not useful before 2 years unless there is a clinical reason (e.g., glucocorticoids, rapid expected change); shorter intervals increase false-positive change.

Bone turnover markers (e.g., CTX, P1NP) can assess adherence and response to therapy but are not diagnostic of osteoporosis.

Monitor treatment response with BMD at spine and hip; a change must exceed the site-specific LSC to be interpreted as real.