What are T-scores (T-scores from Dual-Energy X-ray Absorptiometry) and Z-scores (Z-scores from Dual-Energy X-ray Absorptiometry) from a DEXA (Dual-Energy X-ray Absorptiometry) scan?

Understanding T-Scores and Z-Scores in DEXA Scans

T-scores and Z-scores are standardized measurements from DEXA scans that compare an individual's bone mineral density (BMD) to reference populations, with each standard deviation decrease representing approximately a twofold increase in fracture risk. 1

T-Scores: Definition and Interpretation

• Definition: T-scores represent the number of standard deviations above or below the mean BMD for a young adult reference population 2, 1
• Reference population: Uses the young adult Caucasian female normative reference database for both women and men 2, 1
• Classification system:
  • Normal BMD: T-score ≥ -1.0
  • Osteopenia/low bone mass: T-score between -2.5 and -1.0
  • Osteoporosis: T-score ≤ -2.5 2, 1
• Primary use: Recommended for postmenopausal women, perimenopausal women, and men over age 50 2, 1

Z-Scores: Definition and Interpretation

• Definition: Z-scores represent the number of standard deviations above or below the mean BMD for age and sex-matched controls 1
• Reference population: Uses age, sex, and sometimes ethnicity-matched controls 2, 1
• Classification: A Z-score ≤ -2.0 indicates "bone mineral density below the expected range for age" 2, 1
• Primary use: Recommended for premenopausal women and men under age 50 2, 1
• Clinical significance: Z-scores ≤ -2.0 warrant comprehensive evaluation for secondary causes of bone loss 1

Key Differences Between T-Scores and Z-Scores

1. Reference populations: T-scores compare to young adults at peak bone mass, while Z-scores compare to age-matched peers 1
2. Diagnostic application: T-scores are used to diagnose osteoporosis, while Z-scores identify individuals with BMD lower than expected for their age 2, 1
3. Discordance: Studies show significant differences between T-scores and Z-scores in young adults, with differences up to 1 or more standard deviations in up to 11% of cases 3
4. Clinical impact: Using Z-scores instead of T-scores results in significantly fewer persons being diagnosed with low bone mineral density 4

Clinical Application and Limitations

• Measurement sites: Common sites include lumbar spine, total hip, femoral neck, and one-third radius 1
• Calculation: BMD = bone mineral content (g) ÷ bone area (cm²), expressed as g/cm² 1
• Limitations:
  • Results can vary with different machines, calibration methods, reference standards, and anatomic sites 2
  • Osteoarthritis or aortic calcification may lead to falsely high BMD readings 2
  • Lack of standardization in Z-score calculation methods across manufacturers 3
• Recommendations for follow-up:
  • Serial BMD monitoring should be performed on the same equipment using the same reference standards 2
  • Changes in BMD should be assessed using absolute BMD values (g/cm²), not T-scores or Z-scores 2

Special Considerations

• For children and adolescents, only Z-scores should be used, as the WHO T-score definition of osteoporosis is not applicable 2
• In children, a Z-score < -2 SD indicates BMD less than expected for age, termed "juvenile osteoporosis" 2
• The International Society for Clinical Densitometry (ISCD) recommends that diagnosis of osteoporosis in children should not be made on densitometric criteria alone but should also consider overall bone health and presence of low-trauma fractures 2

Understanding these scores is crucial for proper interpretation of DEXA results and appropriate management of bone health across different age groups and clinical contexts.