The results of your BMD tests will most likely be expressed as a T-score, which uses a mathematical formula and assigns your bone density results either a positive or negative number. Normally, density would be expressed as weight per volume, but because the bone images are only two dimensional, area must be used to calculate your results (grams per centimeter squared [g/cm2]). Because there are different types of machines for BMD testing, a standard way of expressing the bone density measurements was developed.
This method uses a formula comparing your individual results of BMD testing with the mean (average) bone density of healthy young adults of your same sex. The formula is expressed in standard deviations and looks like this:
Patient’s BMD – Young normal mean BMD
Standard deviation of young normal mean
of BMD testing results:
- T-score above –1.0 indicates that your bone mass is normal for your age.
- T-score between –1.0 and –2.5 indicates that you have osteopenia (low bone mass).
- T-score –2.5 or lower indicates that you have osteoporosis.
- T-score –2.5 or lower plus the presence of one or more fragility fractures indicates that you have severe osteoporosis. A fragility fracture is a fracture of a bone that happens with little or no force or trauma, such as falling from your standing height or less.
In 1994, when the WHO developed this method of classification of results, the researchers felt it was important to present results in a way that related them to the lifetime risk of fracture. After all, understanding your risk for breaking bones is a major reason for doing the BMD testing. Bone density is about 10% to 12% lower for each standard deviation below normal (or BMD of 0). Likewise, the risk for fracture doubles with each standard deviation below normal bone density.
So if your T-score is –1.0, your risk of having a fracture is roughly twice that of a young adult. If your T-score is –2.0, your risk is about 4 times higher, and if your T-score is –3.0, your risk for fracture is about 8 times higher than that of a young adult! Figure 9 shows how much more at risk you are as your T-score decreases. But remember, your T-score is only one indicator of fracture risk; other factors—like thinness, age, prior fracture, diet, exercise, and more—also con-tribute to your risk for breaking a bone. The initial development of this method was based on data from White postmenopausal women, making interpretation of results less appropriate for men, pre-menopausal women, or non-White postmenopausal women.
However, data have been collected from men and persons of varied racial/ethnic groups over time. T-scores are now calculated against a mean score developed from young normal people of the same sex as the person being tested. This method is the standard used for all postmenopausal women and men over the age of 50. The International Society for Clinical Densitometry (ISCD) recommends using sex-matched and race- or ethnic-adjusted Z-scores for interpreting BMD test results of premenopausal women, men, and children.
During conventional DXA testing, hip and spine measurements are taken. Depending on the brand of DXA machine, the neck of the femur (upper leg bone) or the point between the neck of the femur and the long part of the upper leg bone (intertrochanteric region) are usually measured for the hip T-score. Sometimes both are measured and the scores are averaged. The lumbar vertebrae (lower back bones) L1 to L4 are measured for the spine T-score. The lower of the two T-scores (hip or spine) is used to classify your degree of bone loss using the WHO guidelines. So, if you have a lower T-score (more bone loss) in your hip than in your spine, your hip T-score will be used for interpreting your results and making the diagnosis.
Table 2 shows an example of the results from a DXA test. Delia’s comment: I’m used to receiving lab results that express a number that I can understand. For example, a white blood cell (WBC) count might come back as 6000 and I know that I have 6000 white blood cells in every little drop of blood, and that’s pretty normal. But when I received the results of my BMD testing of my hip and spine, I was not expecting just a negative number. I was told that my spine was okay. However, I was also told that my T-score was –1.5 and therefore I had low bone mass in my hip bone. What’s even more confusing is that a normal result can still be a negative number, so if I get my bone density to improve enough to be considered normal, I still might have a T-score of –0.9! That doesn’t feel like very good feedback to me!