What do the results of a bone density test mean?

• Your overall bone mineral density score reflects the amount of calcium in your bones.
• The T-score is created by comparing your bone density measurement to that of a healthy young adult at the peak of bone mass, which is achieved in the late 20s or early 30s.
• The Z-score compares your bone mineral density to the average value for someone who is the same age and gender as you.

The results of bone density tests are reported in units called standard deviations (SDs). This is a unit devised by statisticians so they can describe how far a particular value (in this case, your bone density) is from the average value.

Two different scores are used to report the results of bone density tests. First, a T-score compares your particular bone density with the average bone density for a "reference population." In theory, this group consists of "normal young adult women" (as determined by certain criteria set up by researchers). Second, a Z-score compares your particular bone density with that for all women your age.

So, who are these people you're being compared with? The reference population that represents "normal young adult women" with regard to bone density consists of white non-Hispanic women age 20 to 29 who participated in a particular study between 1988 and 1994. These women had dual-energy X-ray absorptiometry (DXA) bone density measurements of the hips, and it is against their values that your values are being compared.

Having bone density that's lower than average is not a disease. It only means you're more likely to have fractures. Having a low bone density measurement is a reason to continue with diagnostic testing for osteoporosis or other bone diseases. It does not, in itself, establish a particular diagnosis or suggest which treatment (if any) you should get.

Who makes up the group of women who have a T-score below -2.5 SDs? Many -- but not all -- of them will have osteoporosis. Some will have osteoporosis because of other medical conditions that require their own treatment plan. Some will have osteomalacia, a metabolic bone disease caused by lack of vitamin D. Some will have no bone disease at all. They have small, thin bones just because that's their normal physiologic makeup.

Who makes up the group of women who have a T-score above -2.5 SDs? Most of these women will not have osteoporosis, but some will. For example, a woman might have above-average bone mass at age 45 but then experience rapid bone turnover. As a result, her bone mass at age 80 may be well below average but not low enough for a T-score of -2.5 SDs.