Osteoporosis is a condition characterized by a loss of bone tissue, causing bones to become porous, brittle, and highly susceptible to fracture. This decrease in bone density significantly increases the risk of breaking bones, even from minor stresses or falls. To ensure consistent diagnosis and effective management, the World Health Organization (WHO) established standardized criteria based on objective measurements of bone mineral density (BMD). These definitions allow healthcare providers to reliably identify individuals who require intervention.
The Diagnostic Tool: DXA Scans
The foundation of the WHO’s diagnostic criteria is the Dual-energy X-ray Absorptiometry (DXA) scan. This non-invasive procedure is the standard for measuring Bone Mineral Density (BMD) and assessing fracture risk. The DXA machine uses a very low dose of X-ray radiation to determine the amount of calcium and other minerals in a specific section of bone.
The results of the scan provide a direct measure of BMD, typically expressed in grams per square centimeter. Measurements are primarily taken at central skeletal sites, most commonly the hip (specifically the femoral neck) and the lumbar spine. These sites are selected because they are frequent locations for fragility fractures and show changes in bone mass earlier. The raw BMD data is then converted into the T-score for diagnostic classification.
Understanding the T-Score System
The T-score is the standardized metric derived from the DXA scan, comparing a patient’s bone density against a specific reference population. It represents the number of standard deviations (SD) the patient’s measured BMD falls above or below the average BMD of a healthy young adult. This reference group represents the peak bone mass typically achieved between the ages of 25 and 35.
The T-score indicates how far a person’s bone health has declined from this optimal peak. A negative T-score signifies lower bone density than the young adult average. Each decrease of one standard deviation below the average approximately doubles the risk for a bone fracture. The T-score is the primary value used for diagnosing osteoporosis in postmenopausal women and men aged 50 years and older.
The WHO Classification Thresholds
The WHO established specific numerical thresholds for the T-score to define categories of bone health. A T-score of -1.0 or higher represents normal bone density, indicating the individual’s BMD is within one standard deviation of the young adult mean.
If the T-score falls between -1.0 and -2.5, the condition is classified as osteopenia (low bone mass). Osteopenia is a precursor to osteoporosis, representing bone density that is below average but does not yet meet the formal criteria for osteoporosis.
The diagnosis of osteoporosis is made when the T-score is -2.5 or lower at the lumbar spine, femoral neck, or total hip. This threshold is clinically significant as it identifies individuals with a substantially increased risk of fragility fractures.
Severe Osteoporosis
For patients meeting the T-score criterion of -2.5 or lower who have already experienced a fragility fracture, the condition is classified as severe or established osteoporosis. This diagnosis highlights that bone weakness has progressed to the point of causing a fracture, often triggering immediate treatment to prevent future breaks.