Bone Density Scan

What is "bone mineral density" (BMD)?

The absolute amount of bone as measured by bone mineral density (BMD) testing generally correlates with bone strength and its ability to bear weight. The BMD is measured with a dual energy x-ray absorptiometry test (referred to as a DXA scan). By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke.

It is important to remember that BMD cannot predict the certainty of developing a fracture. It can only predict risk. It is important to note that a bone density scan, or test, should not be confused with a bone scan, which is a nuclear medicine test that is used to detect tumors, cancer, fractures, and infections in the bone.

The World Health Organization is the source of the commonly-accepted definitions for osteoporosis (WHO Technical Report Series #843, Geneva 1994):

Normal: A value for BMD statistically within 1 standard deviation of the young adult peak bone mass. The report shows a T score > -1 signifying a BMD within the normal range.

Low bone mass (medically termed osteopenia): A value for BMD statistically less than 1 standard deviation but more than 2.5 standard deviations below that of an average young adult. The report shows a T score between -2.5 and £ -1, which signifies an increased fracture risk but does not meet the criteria for osteoporosis.

Osteoporosis: A value for BMD statistically more than 2.5 standard deviations below that of the average peak young adult bone mass. BMD in this range signifies an even higher fracture risk than osteopenia. The report shows a T score £-2.5.

Based on the above criteria, it is estimated that 40% of all postmenopausal Caucasian women have osteopenia and that an additional 7% have osteoporosis (Siris et al JAMA 2001).