What are the definitions of osteopenia and osteoporosis on a dual-energy X‑ray absorptiometry (DEXA) scan?

DEXA Scan Definitions for Osteopenia and Osteoporosis

According to the World Health Organization (WHO) criteria, osteoporosis is defined as a T-score ≤ -2.5, while osteopenia (also termed "low bone mass" or "low bone density") is defined as a T-score between -2.5 and -1.0 at the lumbar spine, femoral neck, total hip, or one-third (33%) radius. 1

T-Score Classification System

The diagnostic classification is based on the lowest T-score at any of the recommended DEXA measurement sites: 1

• Normal BMD: 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

Key Measurement Sites

The primary sites for DEXA measurement include: 1

• Hip (total hip and femoral neck) - with femoral neck designated as the reference site for epidemiological studies 1
• Lumbar spine (L1-L4 region) 1
• Radius (one-third or 33% radius) 1

Important caveat: Forearm measurements should be interpreted with caution as this is not the most relevant site for fracture risk assessment. 1 Additionally, in chronic arthritides such as rheumatoid arthritis, BMD should not be measured at the radius due to local juxta-articular bone loss. 1

Special Population Considerations

Younger Populations (Children, Premenopausal Women, Men <50 years)

For these groups, Z-scores should be used instead of T-scores: 1

• ISCD recommendation: A Z-score ≤ -2.0 is defined as "bone mineral density below the expected range for age," and the terms osteopenia or osteoporosis should not be used in these patients 1
• IOF alternative approach: A T-score ≤ -2.5 in premenopausal women and men younger than 50 years may be viewed as diagnostic of osteoporosis only in the presence of skeletal fragility 1

Clinical Diagnosis Override

A diagnosis of osteoporosis may be presumed in the presence of a prior low-trauma major osteoporotic fracture, even with normal BMD, according to recent pragmatic proposals by EANM, ASBMR, and CSEM. 1 This includes fractures of the hip, spine, forearm, humerus, pelvis, ribs, and tibia (excluding ankle). 1

Important Technical Points

• Diagnostic classification uses the lowest T-score from any recommended skeletal site 1
• T-scores are calculated using young adult Caucasian female normative reference database for both women and men 1
• Serial monitoring should use absolute BMD values (g/cm²), not T-scores or Z-scores 1
• An osteoporotic fracture supersedes any DXA measurement, so patients in the osteopenic range with a fragility fracture should be upgraded to osteoporosis 2

Quantitative CT (QCT) Differences

When using QCT instead of DEXA, different cutoff values apply and the WHO classification does not directly translate: 2

• ACR QCT cutoffs: Low bone mass/osteopenia = 80-120 mg/mL; Osteoporosis = <80 mg/mL 2
• Exception: Projectional QCT of the hip provides areal BMD comparable to DEXA, so the WHO T-score definition of -2.5 for osteoporosis applies 2