What does a T score of -2.9 on a Dual-Energy X-ray Absorptiometry (Dexa) scan indicate?

Interpretation of a T-score of -2.9 on DEXA Scan

A T-score of -2.9 on a DEXA scan indicates osteoporosis according to the World Health Organization (WHO) criteria, as this value falls below the diagnostic threshold of -2.5. 1

Understanding T-scores and Diagnostic Classification

The WHO classification system for bone mineral density (BMD) measurements defines:

• Normal: T-score > -1.0
• Osteopenia (low bone mass): T-score between -1.0 and -2.4
• Osteoporosis: T-score ≤ -2.5 1, 2

A T-score of -2.9 is 2.9 standard deviations below the mean BMD of a young, healthy adult reference population. This clearly falls within the osteoporosis range, indicating significantly decreased bone density and increased fracture risk.

Clinical Significance and Fracture Risk

The clinical importance of a T-score of -2.9 includes:

• Substantially increased fracture risk: Fracture risk approximately doubles with every standard deviation decrease in T-score 1
• Need for therapeutic intervention: This T-score meets the threshold for pharmacologic treatment according to current guidelines 2
• Indication for comprehensive fracture risk assessment: Tools like FRAX should be used to calculate the 10-year probability of major osteoporotic fracture 1

Measurement Site Considerations

When interpreting a T-score of -2.9, it's important to consider which skeletal site was measured:

• Lumbar spine, femoral neck, or total hip: These are the standard measurement sites for diagnosing osteoporosis 2
• Site-specific variations: T-score discordance between measurement sites occurs in approximately 39-45% of cases, with major discordance (one site normal, one site osteoporotic) in about 5% of patients 3
• Diagnostic classification: The lowest T-score from any of the recommended sites (lumbar spine, total hip, or femoral neck) determines the diagnostic classification 1

Age and Gender Considerations

The interpretation of T-scores varies by age and gender:

• Postmenopausal women and men ≥50 years: T-scores are the appropriate reference standard for diagnosis 1, 2
• Premenopausal women and men <50 years: Z-scores (comparison to age-matched peers) are recommended instead of T-scores 1
  • A Z-score ≤ -2.0 is defined as "bone mineral density below the expected range for age" 1

Next Steps After Diagnosis

For a patient with a T-score of -2.9:

1. Initiate pharmacologic treatment: Treatment is clearly indicated for T-scores ≤ -2.5 2
2. Evaluate for secondary causes of osteoporosis:
   • Endocrine disorders (hyperparathyroidism, hyperthyroidism)
   • Gastrointestinal disorders (malabsorption, inflammatory bowel disease)
   • Medication effects (glucocorticoids)
   • Other systemic conditions 2
3. Assess calcium and vitamin D status and ensure adequate supplementation 1
4. Monitor treatment response with follow-up DEXA scans:
   • Typically performed every 2 years
   • Should be done on the same machine as the baseline scan
   • Compare absolute BMD values in g/cm², not just T-scores 1, 2

Common Pitfalls in T-score Interpretation

• Comparing T-scores across different machines: T-scores should only be compared when measured on the same device 1, 2
• Ignoring least significant change (LSC): Changes in BMD must exceed the LSC (typically 2.8-5.6%) to be considered clinically significant 1
• Relying solely on T-scores without clinical context: Fracture risk depends on multiple factors beyond BMD, including age, prior fractures, and comorbidities 2
• Misinterpreting T-scores in younger individuals: In premenopausal women and men <50 years, Z-scores are more appropriate 1, 4

In summary, a T-score of -2.9 represents osteoporosis requiring thorough evaluation and appropriate treatment to reduce fracture risk and associated morbidity and mortality.