## WHO Classification of Bone Mineral Density (BMD) by T-Score **Key Point:** The T-score is the number of standard deviations (SD) a patient's BMD differs from the mean BMD of a healthy 30-year-old adult of the same sex. This is the gold standard for diagnosing osteoporosis. ### WHO Diagnostic Categories | Category | T-Score Range | Clinical Significance | |----------|----------------|----------------------| | Normal | ≥ −1.0 SD | Normal BMD | | Osteopenia | −1.0 to −2.5 SD | Low bone mass; increased fracture risk | | **Osteoporosis** | **≤ −2.5 SD** | Significantly reduced BMD; high fracture risk | | Severe osteoporosis | ≤ −2.5 SD + fragility fracture | Established osteoporosis with fracture | **High-Yield:** The threshold of −2.5 SD was chosen because it represents the fracture threshold in postmenopausal women. Below this value, fracture risk increases exponentially. **Mnemonic:** **NODS** — Normal (≥−1), Osteopenia (−1 to −2.5), Osteoporosis (≤−2.5), Severe (≤−2.5 + fracture) ### Clinical Application **Warning:** A T-score of −2.5 SD is the diagnostic threshold, not −3.5 SD. Severe osteoporosis is defined as a T-score ≤ −2.5 SD **plus** evidence of one or more fragility fractures, not by the T-score alone. **Clinical Pearl:** The T-score is used for postmenopausal women and men ≥50 years. The Z-score (comparison to age-matched controls) is used for premenopausal women and men <50 years to assess whether low BMD is appropriate for age. 
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