A risk factor assessment is an important first step in alerting you to your risk for osteoporosis. However, it is not a scientific way to predict whether your bones will become porous. Many people who develop osteoporosis have few of the risk factors — a fact we are trying to understand through research into the causes of osteoporosis.
This is why medical testing is so important. The tests are known as Bone Mineral Density (BMD) tests and the technology that they use is known as bone densitometry. These tests are safe, painless and accurately measure the density of your bones. A BMD test can tell you whether or not you have osteoporosis and how likely you are to develop it in the future, and can help you to make decisions that may prevent fractures or further bone loss.
When used in combination with important clinical risk factors, a BMD test can help determine your fracture risk and assist your physician in making decisions about your treatment.
BMD testing is not usually recommended in younger men or women (under age 50) unless there is a medical condition predisposing them to osteoporosis or when there are unexplained fragility fractures.
The most common bone density test in use today is called dual energy x-ray absorptiometry (DXA). This test involves lying on a table for several minutes while a small x-ray detector scans your spine, one hip, or both. You may be asked to lift your legs onto a support to straighten your back for the test.
The test is safe and painless and does not require any injections or any other discomfort. You receive a very small amount of radiation from a DXA test, approximately the same amount you are exposed to if you fly from Toronto to Vancouver.
Think of the light shining through the curtains of your home on a sunny day. The amount of light that reaches your eye depends on the thickness of the curtains. If your curtains are very thick, very little sunlight passes through them. If, however, you replace them with a thinner fabric, the light coming through to you will increase substantially.
Similarly, a bone densitometer uses a detector to measure the transmission of small amounts of x-rays (light) through your bones. The amount of light that passes through the bone is measured, thus providing a radiologist with a picture that indicates how dense (thick or thin) your bones are.
In BMD test results, your bones are compared to the bones of an average young adult.
A score (called a T-score) is calculated that describes the density of your bones (usually at the spine and hip) and tells you how strong your bones are compared to this average. While some bone loss with aging is considered normal, making such comparisons helps to determine whether you are losing bone more rapidly than expected for someone your age.
The T-score is expressed in units referred to as “standard deviations (SDs)”. These SD units tell us how far you differ (or deviate) from what is considered normal for a young adult.
Below normal is always indicated with a minus (-) sign. A T-score below 2.5 SDs indicates osteoporosis and a score between 1 and 2.5 SDs indicates “osteopenia” or low bone density.
Normal bone density is no more than 1 SD below the young adult normal value.
Your BMD test result is only part of the picture in determining your risk for fracture. Osteoporosis Canada recommends combining the results of BMD testing with some very important clinical risk factors. This leads to a much better and more accurate method of predicting a person’s risk of fracture.
A comprehensive fracture risk assessment will also lead to a much more accurate determination of a person’s need for prescription osteoporosis medication.