Bone Mineral Density

A measurement used to diagnose osteoporosis. People lose bone mass as they grow older. Typically after the age of 30, existing bone breaks down faster than the body can manufacture new bone, leading to thinning bones (a condition known as osteopenia). Bones also lose calcium and other minerals, making bones lighter, less dense, and more porous. Bones become weaker, increasing the risk of fracture. Eventually, this bone loss may lead to osteoporosis.

Health-care professionals use a bone mineral density (BMD) test to detect osteoporosis, assess the effectiveness of an osteoporosis treatment, and predict the risk of future fractures. There are several methods for estimating BMD. The most accurate is dual-energy x-ray absorptiometry (DEXA). In DEXA, two x-ray beams of different energy levels are used to determine bone density in the spine and hip. The test is fast, painless, and uses minimal amounts of radiation.

Peripheral dual-energy x-ray absorptiometry (pDEXA) is a specific type of DEXA that measures the density of bones in the arms and legs. pDEXA machines are portable and can be used in the doctor’s office. Single-energy x-ray absorptiometry (SXA) is also sometimes used to measure density of bones in the heel or forearm.

Dual-photon absorptiometry (DPA) measures bone density in the hip and spine. It is similar to DEXA, but uses photons (particles of energy) instead of x-rays to produce an image.

Quantitative computed tomography (QCT) is a type of CT scan that measures bone density in the vertebrae (the bones of the spine). A form of QCT called peripheral QTC (pQCT) measures the density of bones in the arms and legs, such as the wrist. QCT is rarely used, however, because it costs more, uses higher doses of radiation, and is less accurate than DPA, DEXA, or pDEXA.

Bone density test results are usually given in the form of a T-score and a Z-score. A T-score compares your BMD with a standard called “young adult normal.” This refers to the average peak bone mineral density of a 30-year-old of your sex. For every standard deviation (about 10% or 12%) difference between your bone density and young adult normal, your T-score rises or falls one point. So a T-score of -2.0 means that your bone density is about 20% lower than young adult normal. Doctors consider a T-score of -1.0 or above to be normal.

Your Z-score compares your BMD to the average for people of your sex and age, though sometimes race and weight are also taken into account.

If your bone density is lower than normal, there are a number of steps you can take to make your bones stronger. These include taking calcium and vitamin D supplements, engaging in weight-bearing exercises such as walking, doing strength training, and taking medicines such as alendronate (brand name Fosamax), ibandronate (Boniva), risedronate (Actonel), and zoledronic acid (Reclast). Calcitonin salmon (Miacalcin) is also used to treat osteoporosis, though an FDA panel recently questioned its ability to reduce a person’s risk of fractures and pointed out that use of the drug may increase a person’s risk of cancer slightly.

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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