What is a T-Score?
A T-score is the number of standard deviations a patient's bone mineral density (BMD) differs from the mean BMD of healthy young adults of the same sex, and it serves as the primary diagnostic criterion for osteoporosis in postmenopausal women and men over age 50. 1
Definition and Calculation
- The T-score compares an individual's BMD measurement to the peak bone mass of a young adult reference population (typically healthy 30-year-old women), expressed as standard deviations above or below this reference mean 2, 3
- The WHO established this metric using the female NHANES reference range, which is now applied to both men and women for diagnostic purposes 1
- The same female reference range and T-score threshold of -2.5 is used for both sexes because the absolute BMD and fracture risk gradient per unit decrease in BMD are similar between men and women 1
WHO Diagnostic Classification System
The T-score directly determines osteoporosis diagnosis according to WHO criteria established in 1994 1, 2:
- Normal bone density: T-score ≥ -1.0 1, 2
- Osteopenia (low bone mass): T-score between -1.0 and -2.5 1, 2
- Osteoporosis: T-score ≤ -2.5 1, 2
Critical Clinical Context
T-scores should only be used in postmenopausal women and men aged 50 years or older—never in premenopausal women, men under 50, or children, where Z-scores (age-matched comparisons) must be used instead 2, 4. This is a common and dangerous pitfall that leads to inappropriate diagnosis and treatment 2.
Measurement Sites and Interpretation
- T-scores are calculated at multiple skeletal sites including lumbar spine, femoral neck, total hip, and one-third radius 2
- The lowest T-score from any of these sites is used for diagnostic classification 2
- However, T-score values can vary significantly between measurement sites due to differences in age-related bone loss patterns and technical factors 5, 6
Limitations You Must Understand
T-scores alone are insufficient for clinical decision-making because they represent only one component of fracture risk 1. Several critical limitations include:
- Most fractures occur in patients with T-scores above -2.5 (in the osteopenic range) simply because this population is much larger, despite their individually lower risk 1
- The same T-score carries different absolute fracture risks depending on age, geography, and other clinical factors 1
- BMD is reasonably specific but not sensitive for identifying individuals who will fracture 1
- Technical factors including machine calibration, measurement site, and presence of osteoarthritis or aortic calcification can falsely elevate readings 1
Integration with FRAX for Treatment Decisions
Rather than treating based solely on T-score thresholds, calculate 10-year fracture probability using the FRAX tool, which incorporates T-score along with clinical risk factors 1. The FRAX algorithm includes:
- Age, sex, and BMI 1
- Prior fragility fracture 1
- Parental history of hip fracture 1
- Current smoking 1
- Glucocorticoid use 1
- Rheumatoid arthritis 1
- Secondary osteoporosis causes 1
- Alcohol consumption 1
- Femoral neck BMD (optional but enhances prediction) 1
Treatment is generally recommended when 10-year hip fracture probability is ≥3% or major osteoporotic fracture probability is ≥20% 4, 7, not simply when T-score reaches -2.5.
Common Pitfalls to Avoid
- Never diagnose osteoporosis in premenopausal women or men under 50 using T-scores 2
- Don't prescribe medication based solely on T-score without calculating FRAX 4
- Don't compare T-scores between different DXA machines or measurement sites when monitoring—compare absolute BMD values instead 4
- Don't ignore the presence of fragility fractures, which may indicate osteoporosis regardless of T-score 2
- Recognize that approximately 50% of fragility fractures occur in patients with osteopenia (T-scores between -1.0 and -2.5), not just those with osteoporosis 4
Monitoring Implications
Serial BMD measurements should be performed on the same DXA scanner using identical protocols, and changes should be assessed by comparing absolute BMD values (g/cm²), not T-scores 1, 4. The appropriate rescreening interval depends on baseline T-score 8: