Q: My mom's bone density scan has improved from -2.87 to -1.03. The doctor said that is good news, but I would like to understand more about what this test shows. Can you help?

Q: My mom's bone density scan has improved from -2.87 to -1.03. The doctor said that is good news, but I would like to understand more about what this test shows. Can you help?

A: I hope the who, what, where, when, why and how (not necessarily in that order) of bone density scans will answer your question.

Why: Fragility fractures are when a bone breaks from fairly minimal trauma. There are many risk factors for bones to break, one of the most important being thin bones (low bone mineral density or BMD).

Some fragility fractures are subtle and may have minimal or no symptoms. For example, vertebral compression fractures (collapse/squishing of the backbones) may manifest only as loss of height (people actually get shorter from this), although back pain from them is also common. Fragility fractures can also have catastrophic consequences; the mortality in the first year after a hip fracture is as high as 20 percent.

People's BMD increases until about age 20, and thereafter gradually decreases. Low BMD is extremely common. In fact, 44 million Americans have osteopenia (low BMD) with another 7 million having osteoporosis (very low BMD to the point where the bones are somewhat porous and fracture much more easily).

It should now be easy to see why measuring BMD may help predict who is at an increased risk for fragility fractures and hence may benefit from treatment.

Furthermore, BMD measurement is useful to follow someone's response to treatment. Interestingly, improved BMD is only one benefit from osteoporosis treatment; one study noted that only about 15 percent of decreased fragility fracture risk from osteoporosis treatment can be attributed to increased BMD.

Who: People at risk for fragility fractures are those who will benefit most from BMD measurements. This includes women older than 65, men older than 75, anyone with a history of a fragility fracture or anyone at high risk for a fragility fracture (such as adults on steroids for more than three months, men with low testosterone, alcoholics, post-menopausal women with a family history of early hip fractures and people over 50 with a tendency to fall, who smoke or who have low body weight).

Other high-risk groups include people with end-stage renal disease, vitamin D deficiency, certain intestinal diseases (such as celiac disease), overactive parathyroids or thyroids, as well as other conditions.

How: BMD is most commonly measured using dual X-ray absorptiometry. With DXA, two different X-ray beams (one better absorbed by bone and the other better absorbed by body tissues) are shot through the patient and the resulting signals analyzed by a computer program to calculate the BMD.

The results are then expressed as a t-score, which is the number of standard deviations (SD, about 15 percent of the average) from average results for a healthy 30-year-old adult. So a t-score of -1 means the patient's BMD is one SD lower than a healthy 30-year-old. A decrease of t-score of 1 (e.g. -1 to -2) doubles the patient's risk of a fragility fracture.

Where: DXA is usually measured on the patient's hip or low spine (lumbar spine), although in select patients other sites such as the wrist may be used. Hip measurements are the most predictive of hip fractures, but otherwise low spine measurements are as predictive.

Different DXA machines and measurement sites may yield variable results. Therefore, having repeat measurements done at the same site (including right or left side) and same machine is best.

When: BMD is measured to assess the patient's risk of fracture, so is usually obtained at age 65 for women, age 75 for men or at an earlier age for anyone at high risk for fragility fractures. It is typically repeated every two years or so to monitor disease progression and/or response to treatment.

What: DXA is considered the gold standard (best test) to measure BMD. A t-score more than -1 is considered normal. T-score less than -1 but more than -2.5 is considered osteopenia, and a t-score less than -2.5 is considered osteoporosis by World Health Organization definitions.

However, what we really care about is the risk of fragility fractures. If you are at risk for a fragility fracture you should have your BMD measured. This result, in addition to consideration of your other risk factors, will help guide your healthcare provider in determining if you should begin treatment to strengthen your bones. See your healthcare provider to discuss this very important issue.

Jeff Hersh, Ph.D., M.D., F.A.A.P., F.A.C.P., F.A.A.E.P., can be reached at DrHersh@juno.com.