What is a T-score in a DEXA Scan?
A T-score represents the number of standard deviations your bone mineral density (BMD) differs from the mean BMD of a healthy young adult reference population (typically aged 25-35 years), and serves as the primary diagnostic criterion for osteoporosis in postmenopausal women and men aged 50 years and older. 1, 2
Understanding the T-score Measurement
The T-score is calculated by comparing your measured BMD value to a young, healthy reference population of the same sex and ethnicity, with the difference expressed in standard deviations. 1, 3 This standardized approach allows for consistent interpretation across different DEXA machines and facilities. 3
WHO Diagnostic Classification
The World Health Organization established the following diagnostic thresholds based on T-scores: 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
For each standard deviation decrease in T-score, fracture risk approximately doubles. 1
Key Differences from Z-score
While T-scores compare your BMD to young healthy adults, Z-scores compare your BMD to age-matched peers of the same sex and ethnicity. 1, 2 In premenopausal women and men younger than 50 years, Z-scores are preferred over T-scores for assessment, with a Z-score ≤ -2.0 considered abnormal. 1, 2 The diagnosis of osteoporosis in younger populations should not be made on BMD testing alone. 1
Clinical Application and Measurement Sites
T-scores are calculated at multiple skeletal sites including the lumbar spine (L1-L4), femoral neck, total hip, and forearm. 1 The lowest T-score from any of these measurement sites is used for diagnostic classification. 2 This approach ensures that osteoporosis is not missed when bone loss is site-specific. 2
Reference Database Considerations
Both men and women should have their T-scores calculated using the young adult Caucasian female normative reference database to maintain consistency with WHO operational definitions. 1 Using different reference databases (such as male-specific databases) will artificially alter T-scores and lead to either underestimation or overestimation of low BMD prevalence. 1 Asian populations may benefit from Asian-specific reference databases for more accurate assessment. 1
Important Clinical Caveats
T-scores should never be used in isolation for treatment decisions—always incorporate clinical risk factors, fracture history, and tools like FRAX for comprehensive fracture risk assessment. 1, 2 Many fractures occur in patients with osteopenia (T-scores between -1.0 and -2.5) rather than frank osteoporosis, highlighting that fracture risk exists on a continuum. 2, 4
A prior low-trauma major osteoporotic fracture (hip, spine, forearm, humerus, pelvis) may indicate osteoporosis even with a normal T-score. 1, 2 In such cases, the clinical diagnosis supersedes the BMD measurement. 1
Monitoring Over Time
When tracking BMD changes, quantitative comparison should be based on absolute BMD values in g/cm² rather than T-scores or Z-scores. 1 T-scores can shift artificially due to software updates or reference database changes, while absolute BMD values provide more reliable longitudinal assessment. 1 Repeat scans should be performed on the same DEXA machine using identical protocols to ensure accurate comparison. 1, 2