How is Bone Density Measured?

There are several ways to evaluate the presence and impact of bone density loss. The World Health Organization (WHO) has established criteria for the diagnosis of osteopenia and osteoporosis based on the relative bone mineral density (BMD) of an individual as compared to a healthy younger population (30 years of age) with normal bone mass. Measuring bone density is essential to the correct diagnosis and effective management of osteoporosis; patients are classified as normal, osteopenic or osteoporotic.

The most commonly used technology to measure bone density is a DEXA scanner (dual energy x-ray absorptiometry), which calculates the BMD at select skeletal sites such as the lumbar spine, hip, and/or wrist, among others. This value is then compared to the BMD in both young and age-matched control populations. The patient is given a T-score, based on comparison to the younger population standard as well as a Z-score, based on comparison to an age-matched healthy population. Different skeletal sites may have different scores. The frequency at which a person should have a bone density test depends on their anticipated rate of bone density loss and the patient's clinical assessment, but is usually every 1 to 3 years. The NOF recommends treatment when a T-score is: less than 1.5 with risk factors present; is less than 2 with no risk factors; and if the person is postmenopausal with a fracture.

The WHO Scale for Evaluating T Scores
T-Score Degree of Bone Loss Fracture Risk
+1 to -1 None None
-1 to -2.5 Osteopenia Moderate to Severe
-2.5 to -3 Osteoporosis Severe
Below -3 Severe Osteoporosis Very Severe