Osteoporosis Classification Based on Bone Mineral Density
Osteoporosis is classified into three distinct stages based on T-scores from DEXA scanning: normal bone mass (T-score ≥ -1.0), osteopenia or low bone mass (T-score between -1.0 and -2.5), and osteoporosis (T-score ≤ -2.5), with a fourth category of severe or established osteoporosis when fragility fractures are present. 1, 2
The WHO Classification System
The World Health Organization established the diagnostic framework that defines osteoporosis stages using T-scores, which represent the number of standard deviations a patient's bone mineral density falls above or below the mean of young, healthy adults 1, 2:
Stage 1: Normal Bone Mass
- T-score ≥ -1.0 1, 2
- Bone density is within normal range compared to young adult reference population 1
Stage 2: Osteopenia (Low Bone Mass)
- T-score between -1.0 and -2.5 1, 2
- Represents decreased bone mass but not yet meeting criteria for osteoporosis 1
- Patients with T-scores below -1.5 have elevated risk for hip and vertebral fractures, which may guide treatment initiation 1
Stage 3: Osteoporosis
- T-score ≤ -2.5 1, 2, 3
- Defined by significantly compromised bone mineral density 2
- Indicates substantially increased fracture risk requiring pharmacological intervention 2
Stage 4: Severe or Established Osteoporosis
- T-score ≤ -2.5 plus one or more fragility fractures 1, 2
- Represents the most advanced stage with documented skeletal fragility 2
- Some experts advocate that two or more fragility fractures should define severe osteoporosis, independent of bone density measurements 4
Critical Clinical Considerations
Fragility Fractures Override T-Score Classification
Any patient with a fragility fracture should be diagnosed with osteoporosis regardless of their T-score. 1 This is crucial because:
- Most fragility fractures occur in patients with T-scores higher than -2.5 1
- A fragility fracture confirms skeletal fragility even when bone density appears preserved 1
- Patients in the osteopenic range who sustain fragility fractures should be upgraded to osteoporosis diagnosis 1
Measurement Sites and Technical Points
- DEXA of the lumbar spine and hip represents the gold standard for diagnosis 1, 2
- Up to 2 vertebral levels may be excluded if falsely elevated by fracture, facet arthritis, or spondylosis 1
- The distal one-third radius serves as an alternative site in hyperparathyroidism or when hip/spine cannot be measured 1
Age and Population Considerations
- These WHO criteria apply specifically to postmenopausal women and men ≥50 years old 1, 2
- Z-scores (comparison to age-matched controls) are used in younger patients to detect secondary causes of osteoporosis 1
Important Pitfall
The T-score threshold of -2.5 has poor sensitivity for identifying patients who will fracture, as the majority of fragility fractures occur above this threshold 1. Therefore, clinical assessment must integrate fracture risk factors beyond bone density alone, including age, prior fractures, family history, glucocorticoid use, and other risk factors captured by tools like FRAX 1, 3.