How do I interpret a bone density test for a patient with a history of fractures, family history of osteoporosis, and medications such as glucocorticoids or anticonvulsants, considering their age, sex, and ethnicity?

How to Interpret a Bone Density Test

Interpret bone density using T-scores for adults ≥50 years (osteoporosis = T-score ≤-2.5) and Z-scores for younger adults (low bone mass = Z-score ≤-2.0), while integrating FRAX calculations and clinical risk factors—particularly prior fractures, glucocorticoid use, and family history—to guide treatment decisions rather than relying on BMD values alone. 1, 2

Understanding the Scoring Systems

T-Score Interpretation (Adults ≥50 Years)

• T-score ≤-2.5 at the femoral neck, total hip, or lumbar spine establishes the diagnosis of osteoporosis 1, 3
• T-score between -1.0 and -2.5 indicates osteopenia (low bone mass) 1
• T-score ≥-1.0 represents normal bone density 1
• T-scores measure the number of standard deviations below the mean BMD for healthy young adults and represent current fracture risk 4

Z-Score Interpretation (Adults <40-50 Years)

• Z-score ≤-2.0 is defined as "low BMD for chronological age" or "below the expected range for age" 1
• Z-score >-2.0 is "within the expected range for age" 1
• Use Z-scores rather than T-scores in younger adults to detect secondary causes of bone loss 3
• Z-scores compare BMD to age-matched controls and measure fracture risk relative to peers 4

Critical Clinical Context Integration

Prior Fractures (Highest Priority)

• Any fragility fracture after age 50 is diagnostic of osteoporosis regardless of BMD and is the strongest predictor of future fractures 3
• Presence of any prevalent fracture increases relative risk for future fractures by approximately 2-fold or more 5
• Self-reported but undocumented prior vertebral fracture warrants vertebral fracture assessment if T-score <-1.0 1

Glucocorticoid Use Adjustments

• Adjust FRAX calculations by multiplying major osteoporotic fracture risk by 1.15 and hip fracture risk by 1.2 if prednisone dose >7.5 mg/day 1
• For patients on ≥7.5 mg/day prednisone equivalent for ≥3 months, consider treatment at higher BMD thresholds 1
• Very high-dose glucocorticoids (prednisone ≥30 mg/day, cumulative dose >0.5 gm/year) require more aggressive monitoring and earlier treatment 1

Family History Impact

• Family history of hip fracture (particularly parental) is a major risk factor that should lower your treatment threshold 1
• This factor is incorporated into FRAX calculations but should be explicitly documented 2

FRAX Integration for Treatment Decisions

When to Use FRAX

• Calculate 10-year fracture probability using FRAX for all adults ≥40 years with risk factors or osteopenia 1, 2
• FRAX is particularly valuable in patients with osteopenia (T-score -1.0 to -2.5) to determine if pharmacologic treatment is warranted 2
• Include BMD in FRAX calculation when available, as this improves predictive accuracy 2

Treatment Thresholds

• Recommend treatment if 10-year risk of hip fracture ≥3% OR major osteoporotic fracture ≥20% 2, 3
• These thresholds apply to adults ≥40 years not already on treatment 2
• FRAX is not validated for adults <40 years; use clinical risk assessment with BMD testing instead 2

Age-Specific Interpretation Algorithms

Adults ≥65 Years

• Screen all women ≥65 years and men ≥70 years regardless of other risk factors 1
• Treatment decisions should integrate T-scores, FRAX calculations, and clinical risk factors 1, 2
• Consider Z-scores when evaluating for secondary causes of bone loss even in this age group 3

Adults 40-64 Years

• Perform BMD testing if one major risk factor (prior fracture, family history of hip fracture, glucocorticoid use) or two minor risk factors exist 1, 5
• Use FRAX with BMD to guide treatment decisions 1, 2
• Minor risk factors include low body mass index, low calcium intake, alcohol consumption ≥3 units/day, and smoking 1

Adults <40 Years

• Use Z-scores, not T-scores, for interpretation 1, 3
• Perform BMD testing only if history of osteoporotic fracture, Z-score <-3, >10%/year BMD loss, very high-dose glucocorticoids, or other significant risk factors 1
• Focus on identifying secondary causes of bone loss (hypogonadism, malabsorption, hyperparathyroidism, hyperthyroidism) 1, 3

Medication-Specific Considerations

Anticonvulsants

• Anticonvulsants are associated with increased fracture risk and accelerated bone loss 1
• Lower your treatment threshold in patients on chronic anticonvulsant therapy 1
• Consider more frequent monitoring (every 1-2 years) 1

Other High-Risk Medications

• Aromatase inhibitors, cancer chemotherapeutic drugs, and gonadotropin-releasing hormone agonists all increase fracture risk 1
• Anticoagulants are also implicated in increased fracture risk 1

Ethnic and Sex Considerations

Sex Differences

• Men account for 24% of hip fractures and 21% of wrist fractures but only receive 10% of BMD tests—this represents significant underutilization 5
• For men with osteoporosis, treatment with bisphosphonates reduces vertebral fracture risk 1
• Men with prostate cancer on androgen deprivation therapy require screening and 1-2 year follow-up intervals 1

Ethnicity

• White persons are at higher risk than Black persons 1
• FRAX has limitations regarding race-specific calculations, which may lead to differences in treatment recommendations 2
• Use the reference data for the patient's ethnicity when available 1

Common Pitfalls to Avoid

DXA Limitations

• DXA measures areal BMD (mass per unit area), not true volumetric density, which may underestimate bone density in short individuals or overestimate in tall individuals 1
• BMD identifies less than half of people who will have an osteoporotic fracture—clinical risk factors are essential 1
• Artifacts from degenerative changes, aortic calcification, or vertebral fractures can falsely elevate lumbar spine BMD 1

Interpretation Errors

• Never rely on BMD alone—a patient with T-score -2.0 and prior fragility fracture has osteoporosis and requires treatment 3
• Do not use T-scores in adults <50 years; this leads to overdiagnosis 1, 3
• Do not ignore secondary causes: 44-90% of cases in premenopausal women and men <50 years have identifiable secondary causes 3

Follow-Up Testing Mistakes

• Do not perform serial BMD testing more frequently than every 2-3 years in most patients 1
• For low-risk individuals with normal baseline BMD (T-score ≥-1) and bone loss <1%/year, repeat testing is not necessary for at least 3 years 5
• During the first 5 years of osteoporosis treatment, routine BMD monitoring is not recommended as changes in BMD explain only a small percentage of fracture risk reduction 1, 5

Site-Specific Measurement Priorities

Preferred Sites

• Measure both lumbar spine (L1-L4) and hip (femoral neck and total hip) when feasible 1, 3
• Use the lowest T-score from any site (lumbar spine, femoral neck, or total hip) for diagnosis 1
• The posteroanterior lumbar spine is optimal for monitoring treatment response 4

Special Populations

• In spinal cord injury patients, measure total hip, proximal tibia, and distal femur 1
• Avoid lumbar spine measurements if significant degenerative changes are present 1