What are the diagnostic criteria for osteoporosis?

Diagnostic Criteria for Osteoporosis

Osteoporosis is diagnosed when a dual-energy X-ray absorptiometry (DXA) scan shows a T-score of -2.5 or lower at the lumbar spine, femoral neck, total hip, or one-third (33%) radius, according to the World Health Organization (WHO) criteria. 1, 2

T-Score Classification System

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

T-score	Diagnosis
> -1.0	Normal
-1.0 to -2.4	Osteopenia (low bone mass)
≤ -2.5	Osteoporosis

Clinical Diagnostic Criteria

Beyond DXA measurements, osteoporosis may also be diagnosed in the following scenarios:

• Prior fragility fracture: A diagnosis of osteoporosis may be presumed in the presence of a prior low-trauma major osteoporotic fracture (hip, spine, forearm, humerus, pelvis, ribs, tibia excluding ankle), even with normal BMD 2, 1

• T-score measurement site: Diagnostic classification is based on the lowest T-score at any of the recommended DXA regions 2

Special Populations

1. Premenopausal women and men under 50:

   • ISCD recommends using Z-scores instead of T-scores 2
   • Z-score ≤ -2.0 is defined as "bone mineral density below the expected range for age" 2
   • IOF recommends that a T-score ≤ -2.5 may be viewed as diagnostic of osteoporosis in the presence of skeletal fragility 2

2. Men over 50:

   • The same absolute T-score values used in women can be used in men 3
   • Use of the young adult Caucasian female normative reference database for T-score calculation is recommended for both women and men 2

DXA Measurement Considerations

• DXA is the gold standard for diagnosing osteoporosis and monitoring changes in BMD over time 4, 2
• Measurements should be performed at the lumbar spine and hip as primary sites 2, 1
• For patients with advanced degenerative changes of the spine or scoliosis, QCT of the lumbar spine and hip may be more appropriate (rated 8/9 for appropriateness) 2
• The distal forearm (one-third radius) should be measured when the hip or spine cannot be accurately assessed 2

Fracture Risk Assessment

While T-scores are used for diagnosis, fracture risk assessment tools provide additional information:

• FRAX tool: Calculates 10-year probability of major osteoporotic fracture and hip fracture 2, 1
• Treatment thresholds: Consider treatment with osteopenia if FRAX indicates 10-year hip fracture risk ≥3% or major osteoporotic fracture risk ≥20% 1

Common Pitfalls in DXA Interpretation

• Using incorrect reference databases for T-score calculation 5
• Improper patient positioning during scan acquisition 6
• Comparing follow-up BMD measurements using T-scores rather than absolute BMD values in g/cm² 2, 1
• Not accounting for artifacts or degenerative changes that may artificially elevate BMD measurements 2
• Failing to use the same DXA machine, software, scan mode, and patient positioning for follow-up scans 2

Monitoring Considerations

• Follow-up DXA should be performed on the same machine as the baseline scan 2, 1
• Compare absolute BMD values in g/cm², not T-scores or Z-scores 2, 1
• Each facility should calculate its own least significant change (LSC) value (typically 2.8-5.6%) 2, 1

By adhering to these diagnostic criteria and measurement principles, clinicians can accurately identify patients with osteoporosis who may benefit from pharmacologic therapy to reduce fracture risk and associated morbidity and mortality.