Clinical Densitometry Knowledge and Skills
What the exam expects
This domain tests whether you can move correctly from scan acquisition to interpretation. Know the technology distinctions first: DXA uses two x-ray energies to separate bone from soft tissue; QCT measures volumetric density in mg/cm³; pDXA and QUS are peripheral tools with limited monitoring roles. Understand T-score versus Z-score. The T-score compares a patient to a young-adult reference mean in standard deviations and drives diagnosis in postmenopausal women and men 50 and older. The Z-score compares to an age-, sex-, and ethnicity-matched reference and is preferred in premenopausal women, men under 50, and children; a Z-score of −2.0 or lower warrants evaluation for secondary causes.
Know the WHO categories: normal T-score −1.0 or above, osteopenia (low bone mass) between −1.0 and −2.5, osteoporosis at −2.5 or below. Recall that the diagnosis uses the lowest T-score among valid sites (lumbar spine, total hip, femoral neck, 33% radius), and that the forearm is used when spine and hip are not interpretable. Understand precision: the least significant change (LSC) at 95% confidence is roughly 2.77 times the precision error, and a change must exceed the LSC to be real. Monitoring intervals should be set so that expected change exceeds the LSC.
Traps
Questions love to swap Z for T in a young patient, apply a T-score to a peripheral device for diagnosis, or treat a change smaller than the LSC as meaningful. Watch for artifact exclusions: vertebroplasty, metal hardware, severe scoliosis, and degenerative changes inflate spine BMD; exclude those vertebrae rather than the whole scan. Also remember that BMD alone does not equal fracture risk-clinical risk factors and FRAX belong in the assessment.
Rule of thumb
If the patient is a postmenopausal woman or a man 50 or older, report and diagnose with the T-score; otherwise lead with the Z-score.