Understanding Bone Density Scan
Q.I recently had a bone density scan. I was told the results were -1, which indicates some bone loss. Since then I heard that the -1 figure actually indicates a 10 percent loss of bone density. Is this correct?
A.People who are at high risk for osteoporosis can receive a test called a DEXA (dual energy X-ray absorptiometry) scan. This is currently the best test available for detecting the approach of osteoporosis.
A.The DEXA is a noninvasive, painless type of X-ray that scans bones in the hip and the lumbar spine (the lower part of the back). Technicians then plot the patient’s bone density on a graph. One score, called the T-score, compares the patient’s bone density to that of someone in his or her early 30s, the age when bone density is at its peak. A second score, called the Z-score, compares the patient’s bone density to that of people in his or her own age group. Both scores show how far above or below the norm a person falls, or the standard deviation. Your result of -1 means that you are one standard deviation below the average.
Your doctor makes the final interpretation of the DEXA scan. In general, if a patient’s score falls more than two standard deviations below the average at the age of peak bone mass (T-score), then that person is considered to have osteoporosis. For results that are only one or two standard deviations below the average, doctors use the term osteopenia, meaning that there is some loss of bone but not severe enough to be called osteoporosis.
A.Although Z-scores are helpful, showing where the patient falls in relation to people of similar age and sex, the T-score is most important. It shows how much bone density has been lost. If there has been significant loss of bone density, treatment should be started.
It is difficult to give percentages for how much bone has been lost based on the DEXA numbers. The main concern is to see how the patient’s bone density compares to that of peak bone density years. Furthermore, one important benefit of the DEXA scan is that it can be used sequentially. In other words, in a year or so you can repeat the DEXA scan to see if treatment has helped.