Bone density often decreases in people over age 50. Over 8 million women and 2 million men in the United States have osteoporosis, thin bones that increases the risk of bone fracture, according to the Merck Manuals Online Medical Library. The most common way to assess bone density is with Dual Energy X-ray Absorptiometry, often simply called a DEXA or DXA scan. Measurements taken at the hip and spine are compared to both the bone density of a typical 30-year-old of your same sex, called a T-score and to other people of your age, called a Z-score.
T-Score
Since a T-score compares your bone density to that of a healthy 30-year-old, there's more likely to be a difference between your levels and the standard. Results are reported in terms of standard deviations. A score of 0 means that your bone density is the same as that of a normal 30-year-old. Negative levels indicate lower than normal bone density. A T-score of -1 is considered within normal limits; a T-score of -1 to -2.5 indicates low bone mass, or osteopenia. A T-score greater than -2.5 is diagnosed as osteoporosis.
Z-Score
The Z-score compares your bone density to the bone density of people of your age, sex and sometimes gender and racial background, using standard deviations. A Z-score is not used to diagnose osteoporosis, but rather to determine if your bone density is low for reasons other than primary osteoporosis related to age. Other underlying disease processes can also cause osteoporosis and might require different treatment. A Z-score lower than -1.5 may indicate a secondary cause for osteoporosis, MayoClinic.com reports.
Fracture Risk
The DXA test is currently the single best predictor of fracture risk, states the National Institute of Arthritis and Musculoskeletal and Skin Diseases, with a low T-score indicating an increased fracture risk. The bone density levels at the hip and spine in an individual usually have about the same relative value, according to Susan Ott, M.D of the University of Washington. However, if levels differ, the T-score specific to the location applies. You may have a higher risk of hip or spine fracture based on your T-score.
Considerations
Bone loss from osteoporosis normally causes no symptoms until a fracture occurs. Taking calcium and vitamin D supplements and getting adequate weight-bearing exercise can help prevent bone loss as you age. Medication called bisphosphonates can help improve bone density or decrease bone loss once you develop osteoporosis, which reduces the risk of fractures, particularly in the spine and hip. Taking testosterone may help improve bone density in men who have low testosterone levels.
References
- National Institute of Arthritis and Musculoskeletal and Skin Diseases; What Is a Bone Density Test?; May 2009
- MayoClinic.com; Bone Density; October 2009
- University of Washington; Bone Density; June 2008
- University of Washington; Bone Density and Fracture Risk; July 2007
- Merck Manuals Online Medical Library; Osteoporosis; Lawrence Raisz, M.D.; February 2008
