Bone mineral density (BMD) tests are the only way to detect osteoporosis. These tests are painless and noninvasive, and they are very helpful in terms of measuring bone strength. There are a number of tests available: central machines, which measure BMD in the hip, wrist, spine, or total body; and peripheral machines, which measure BMD in the finger, hand, forearm, heel, or shin bone.
The most widely used and best understood BMD test is DEXA (Dual Energy X-ray Absorptiometry). DEXA measures BMD of the spine, hip, or total body. While radiation is used to conduct a DEXA scan, it involves approximately a tenth of the radiation required for a single chest x-ray. To conduct this test, the patient rests on a padded table. A large mechanical arm moves over the body, with a total body scan taking approximately 20 minutes to complete.
Results of BMD tests are usually expressed as "T-scores." A T-score involves a statistical term called "standard deviation," a measurement of how far something is away from what is considered "normal." With respect to BMD testing, "normal" has been defined as the average BMD of healthy, young controls (representing peak bone density). If you are a female undergoing BMD testing, your bone density will be compared to that of a population of young, healthy women; if you are a male undergoing BMD testing, your bone density will be compared to that of young, healthy men. A T-score is the number of standard deviations below (or above) these "normal" cases:
- Normal bone: T-score better than -1.
- Osteopenia: T-score between -1 and -2.5 (discussed in the next section)
- Osteoporosis: T-score less than -2.5
BMD testing will also produce what is known as a "Z-score." This compares the BMD of the person being evaluated with the average BMD of people of the same age and sex. However, when it comes to diagnosing osteoporosis, the T-score is of the most importance.
At the present time, there aren't any official guidelines regarding BMD testing in HIV-positive people. Very few, if any, experts believe that all HIV-positive people should undergo routine BMD testing. Experts do agree that some HIV-positive people, including those who have been infected with HIV for many years and those who have other established risk factors for osteopenia/osteoporosis (see: "What are the risk factors for osteoporosis?"), may want to consider BMD testing. Hopefully, official guidelines will soon be established to guide this decision.
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