Osteoporosis is a skeletal disorder characterized by compromised bone disorder predisposing to an increased risk of fracture. It occurs when your body produces too little bone, there is a reduction in bone mass due to aging or other causes, or both (Figure 1).
The primary diagnostic test for osteoporosis is bone mineral density (BMD). This test, which is generally taken at the proximal femur (hip) and spine (Figure 2), helps estimate the density of bones and the likelihood of breaking a bone. The hip, particularly the femoral neck, and spine are used because these are the most common sites for fractures. BMD is measured by a dual-energy X-ray absorptiometry (DXA). DXA testing provides an estimate of real BMD in g/cm2, and the estimate is converted into a T-score by comparing it to the BMD levels of a healthy young adult population. The BMD values of the 20- to 29-year-old females from the NHANES-III study (1988 to 1994) population are typically used as the reference population. Using thresholds developed by the World Health Organization (WHO), osteoporosis is defined as a T-score of -2.5 or less at either the lumbar spine or proximal femur (hip). T-scores between -2.5 and -1.0 identify individuals with low bone mass. T-scores greater than -1.0 represent normal bone mass.1
When BMD measurements are not available, diagnosis of osteoporosis is sometimes made based on fragility fractures, particularly with respect to the spine. The presence of vertebral fracture (VF) identifies a patient who has clinical osteoporosis; however, up to 75% of VFs are asymptomatic. Lifetime height loss of 1.5 inches or more also can be a sign of osteoporosis when BMD does not indicate osteoporosis or vertebral fractures are not present. One study found that 30% of men and women would have been misclassified (undiagnosed with osteoporosis) based on bone mineral density alone.2