DXA bone scan

What it measures

Dual-energy X-ray absorptiometry (DXA) measures bone mineral density, the primary diagnostic tool for osteoporosis - expressed as a T-score (comparison to a young healthy reference population) used to diagnose osteoporosis/osteopenia and to guide treatment decisions and monitor response
The standard investigation for osteoporosis diagnosis and monitoring, widely used given its low radiation dose, reasonable cost, and reproducibility for serial monitoring
Bone mineral density measured by DXA correlates with (though does not perfectly predict) fracture risk - bone strength depends on both density and microarchitectural quality, which DXA does not directly assess, explaining why fracture risk assessment tools (e.g. FRAX) incorporate clinical risk factors alongside the DXA result rather than relying on T-score alone

T-score (comparison to young healthy adults) is used for diagnosis: normal (T-score ≥ -1.0), osteopenia (-1.0 to -2.5), osteoporosis (≤ -2.5); Z-score (comparison to age-matched peers) is more relevant in premenopausal women and younger patients, where a low T-score against a young reference population would over-diagnose age-appropriate bone density
DXA is typically performed at the lumbar spine and hip (the sites with the strongest fracture-risk correlation and treatment trial evidence)

Diagnostic T-score, combined with clinical fracture risk factors (often via FRAX or similar tools), determines pharmacotherapy initiation threshold rather than DXA result alone
Serial DXA monitoring (typically at 1-2 year intervals once treatment starts) tracks treatment response, though bone density changes are gradual and short-interval repeat scanning is not usually informative

Osteoporosis diagnostic threshold (T-score ≤ -2.5), FRAX fracture risk integration, Z-score use in premenopausal/younger patients, treatment monitoring interval

A T-score in isolation should not be the sole determinant of treatment decisions - fracture risk assessment tools combining the DXA result with clinical risk factors (age, prior fracture, glucocorticoid use, family history) give a more complete risk picture than bone density alone, since bone strength depends on more than density alone

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