Understanding a bone density studyPosted: February 3, 2012 | Author: onhc | Filed under: Blog | Comments Off on Understanding a bone density study
The report from a bone density study, or a DEXA (dual-energy X-ray absorptiometry) scan, can be extremely confusing. It is short on basic information for the patient and long on graphs and numbers. Since most physicians don’t have time to explain all those numbers, patients often walk out with a copy of their report and a head full of questions. If you have been diagnosed with osteopenia or osteoporosis, you’ll want to increase the density of your bones, and understanding your DEXA results will help you monitor your progress.
DEXA reports will vary depending on the type of machine used for the study. There are common components however, that are critical to interpretation. When you get your report, look for the following columns: BMD, T-score, and Z-score. These are the most important numbers on your report. The column marked BMD gives you your bone mineral density in grams per centimeter of bone. The rest gets slightly more complicated.
Since the normal population with the greatest risk for osteoporosis is postmenopausal women, the T-score measures how far your bone density deviates from the average bone density of healthy young adult females (the age at which bone density has peaked). If your bone density is equal to the average bone density in this population then your T-score will be 0 (no deviation). Each standard deviation is about 10-12% of bone mineral density and is equal to a T-score of 1, so if your bone density is 20% lower than the average bone density of young adult women (two standard deviations), your T-score will be -2.0. The World Health Organization has set the following guidelines: A T-score between -1.0 and +1.0 is said to be “normal.” A T-score of -1.0 to -2.5 indicates osteopenia, or low bone density, and a T-score that falls below -2.5 indicates osteoporosis, more severe bone loss. Each standard deviation below normal approximately doubles the risk for fracture, so someone with a T-score of -2.0 has approximately 4 times the risk of bone fracture as that of a young adult female.
The Z-score is also a standard deviation measure, but this comparison is age, race, and sex matched. If you are a 75 year old African American woman, your Z-score determines how your bone density compares to that of others in this population. The Z-score is important, but doesn’t tell us much about the actual strength of the bone. The average 80 year old white woman has weak bones, even if her Z-score is 0. By comparing your bone density to the average bone density of your matched population, a low score might indicate that there is a reason other than age related bone loss. A low Z-score will often prompt a physician to do more testing to rule out other causes of bone loss.
There are myriad laborious details to fully interpreting a DEXA which goes well beyond the scope of a blog. Hopefully the nuts and bolts of it, explained here, will help you better understand your results and monitor the strength of your bones.