Osteoporosis is a systemic skeletal disorder defined by compromised bone strength, which significantly increases the risk of breaking a bone from minor trauma. To ensure consistent diagnosis and allow for data comparison worldwide, the World Health Organization (WHO) established standardized criteria in 1994. This universal framework classifies a patient’s bone health status and relies on quantitative measurement of bone density to accurately assess fracture risk.
Measuring Bone Density: The DEXA Scan
The necessary data for the WHO classification is obtained through a Dual-energy X-ray Absorptiometry (DEXA) scan, which is considered the gold standard for measuring Bone Mineral Density (BMD). The DEXA machine sends two distinct X-ray beams through the body, and the detector measures how much radiation passes through, allowing for the calculation of mineral content in grams per square centimeter.
The scan focuses on measuring BMD at specific sites in the central skeleton, as changes here are often a better indicator of overall fracture risk. The standard skeletal sites measured are the lumbar spine and the proximal femur, including the total hip and the femoral neck. The BMD results from these specific areas are then processed by the machine’s software to generate the numerical scores used in the WHO criteria.
The WHO T-Score Diagnostic Categories
The primary measurement used by the WHO for diagnosing osteoporosis in postmenopausal women and men aged 50 and older is the T-score. The T-score is a statistical value that compares an individual’s measured BMD to the average BMD of a healthy young adult reference population at peak bone mass. The result is expressed as the number of standard deviations (SD) the patient’s score falls above or below this young adult mean.
A T-score greater than or equal to \(-1.0\) is classified as Normal bone density, meaning the patient’s bone mass is within one standard deviation of the young adult reference. The next category is Low Bone Mass, often referred to as osteopenia, which is defined by a T-score that falls between \(-1.0\) and \(-2.5\). This indicates that the bone density is lower than normal but has not yet reached the established threshold for osteoporosis.
The diagnosis of Osteoporosis is made when the T-score is at or below \(-2.5\). This numerical threshold signals a significant loss of bone mass and a substantially increased risk of fragility fractures. The most severe classification is Established or Severe Osteoporosis, which is defined as a T-score of \(-2.5\) or below combined with the patient having already experienced one or more fragility fractures.
For premenopausal women, men under 50, and children, the T-score is not used for diagnosis. In these cases, the Z-score is reported, which compares the patient’s BMD to that of an age-matched and sex-matched reference population. The Z-score is used to assess if bone density is lower than expected for their age, prompting a search for underlying medical causes.
Beyond Diagnosis: Clinical Application of the Criteria
The WHO criteria guide clinical decision-making regarding screening and treatment. The classification helps medical professionals determine who should be screened with a DEXA scan. General guidelines recommend testing for all women aged 65 and older and men aged 70 and older. Younger individuals with specific risk factors, such as a prior fragility fracture or certain medical conditions, are also candidates for screening.
The T-score category is directly applied to treatment decisions, establishing a clear line for when pharmacological intervention may be necessary. While a diagnosis of osteoporosis (T-score \(\leq -2.5\)) warrants treatment, the low bone mass (osteopenia) category is more complex. Many fractures occur in patients with osteopenia, so BMD alone is often insufficient to guide treatment.
To address this, the WHO developed the Fracture Risk Assessment Tool (FRAX). FRAX integrates the T-score from the femoral neck with several other clinical risk factors like age, body mass index, and prior fracture history. FRAX calculates the individual’s 10-year probability of a major osteoporotic fracture or a hip fracture. This combined risk assessment allows for targeted treatment of individuals with osteopenia who are at a high risk of fracturing a bone.