When is the Z score used in the interpretation of bone density Dual-Energy X-ray Absorptiometry (DEXA) results?

When to Use Z-Score in DEXA Interpretation

Z-scores should be used instead of T-scores in premenopausal women, men under age 50, and children/adolescents, as WHO diagnostic criteria for osteoporosis do not apply to these populations. 1

Primary Populations Requiring Z-Score Interpretation

Premenopausal Women

• Only Z-scores (not T-scores) should be reported in premenopausal women, regardless of bone density level 1
• Z-scores represent gender- and age-matched controls for evaluation of secondary osteoporosis 1
• A Z-score ≤ -2.0 is defined as "below the expected range for age" 1
• A Z-score > -2.0 is "within the expected range for age" 1

Men Under 50 Years of Age

• A diagnosis of osteoporosis cannot be made in men <50 years of age on the basis of BMD alone 1
• Z-scores must be used for this population instead of T-scores 1
• The same Z-score thresholds apply as for premenopausal women 1

Children and Adolescents

• Z-scores are recommended for children and adolescents 2
• Diagnosis should not be based solely on densitometric criteria but should include assessment of overall bone health and presence of fractures 2

Clinical Scenarios Where Z-Scores Are Essential

Secondary Osteoporosis Evaluation

When evaluating for secondary causes of bone loss in younger patients with:

• Chronic renal failure 1
• Rheumatoid arthritis and inflammatory arthritides 1
• Eating disorders (anorexia nervosa, bulimia) 1
• Organ transplantation 1
• Prolonged immobilization 1
• Gastrointestinal malabsorption or malnutrition 1
• Endocrine disorders (hyperthyroidism, Cushing syndrome) 1

Medication-Related Bone Loss

Z-scores should be used for younger patients receiving:

• Glucocorticoid therapy for >3 months 1
• Androgen deprivation therapy 1
• Aromatase inhibitor therapy 1
• Chronic heparin 1
• Anticonvulsant drugs 1

Important Distinctions: When T-Scores Are Used Instead

Postmenopausal Women (Any Age)

• T-scores should be used for all postmenopausal women, regardless of age 2, 3
• This includes women with premature or early menopause 3
• WHO diagnostic criteria apply: T-score ≤ -2.5 indicates osteoporosis 1, 2

Men Age 50 and Older

• T-scores are appropriate for men ≥50 years 2
• WHO criteria can be applied to this population 1

Critical Interpretation Points

Z-Score Calculation Considerations

• Z-scores should be population-specific where adequate reference data exist 1
• Patient's self-reported ethnicity should be used in Z-score calculations 1
• Caution: Z-score definitions are not standardized across DXA manufacturers, leading to potential discordance 4
• Differences between Z-scores and T-scores can be substantial in young adults (up to 1 SD or more in 11% of cases) 4

Common Pitfalls to Avoid

• Never use T-scores to diagnose osteoporosis in premenopausal women or men <50 years 1
• Do not apply WHO T-score criteria to populations where Z-scores are indicated 1
• Be aware that Z-scores have less diagnostic value for osteoporosis compared to T-scores in appropriate populations 5
• Remember that Z-score interpretation should prompt investigation for secondary causes of bone loss when values are below expected range 1

Clinical Decision Algorithm

1. Determine patient category first:

   • Postmenopausal woman (any age) → Use T-score 2, 3
   • Man ≥50 years → Use T-score 2
   • Premenopausal woman → Use Z-score 1
   • Man <50 years → Use Z-score 1
   • Child/adolescent → Use Z-score 2

2. If Z-score is indicated and ≤ -2.0:

   • Classify as "below expected range for age" 1
   • Investigate for secondary causes of bone loss 1
   • Consider evaluation every 2-3 years if on glucocorticoids 1