A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray or computed tomography (CT) scan. This information is used to estimate the strength of your bones.

We all lose some bone mass as we age. Bones naturally become thinner (called osteopenia) as you grow older because existing bone is broken down faster than new bone is made. As this occurs, our bones lose calcium and other minerals and become lighter, less dense, and more porous. This makes the bones weaker and increases the chance that they might break (fracture).

With further bone loss, osteopenia leads to osteoporosis . So the thicker your bones are, the longer it takes to get osteoporosis. Although osteoporosis can occur in men, it is most common in women older than age 65.

If your bone density is lower than normal, you can take steps to increase your bone strength and reduce your chances of having a fracture. Some ways to increase bone density and strength include combining calcium and vitamin D supplements with weight-bearing exercise (such as walking), weight training (such as lifting weights or using weight machines), and using medicines.

There are several different ways to measure BMD.

• Dual-energy X-ray absorptiometry (DEXA). This is the most accurate way to measure BMD. It uses two different X-ray beams to estimate bone density in your spine and hip. Strong, dense bones allow less of the X-ray beam to pass through them. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to each other. DEXA can measure as little as 2% of bone loss per year. It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to measure heel and forearm bone density, but SXA is not used as often as DEXA. See a picture of a DEXA X-ray of the hips or a DEXA X-ray of the spine .
• Peripheral dual-energy X-ray absorptiometry (P-DEXA). P-DEXA is a type of DEXA test. It measures the density of bones in the arms or legs, such as the wrist—it can't measure the density of the bones most likely to break, such as the hip and spine. P-DEXA machines are portable units that can be used in a doctor's office. P-DEXA also uses very low doses of radiation, and the results are ready faster than standard DEXA measurements. P-DEXA is not as useful as DEXA for finding out how well medicine used to treat osteoporosis is working.
• Dual photon absorptiometry (DPA). This test uses a radioactive substance to measure bone density. It can measure BMD in your hip and spine. DPA also uses very low doses of radiation but has a slower scan time than the other methods.

Ultrasound is a screening test that is sometimes offered at events such as health fairs. It is only used to look for problems. If results from an ultrasound test find low bone density, DEXA is recommended to confirm the results. Ultrasound uses sound waves to measure BMD, usually in your heel. Ultrasound is quick, painless, and does not use potentially harmful radiation like X-rays. One disadvantage of ultrasound is it can't measure the density of the bones most likely to fracture from osteoporosis (the hip and spine). Ultrasound is not used to keep track of how well medicine for osteoporosis is working.

Before being screened for osteoporosis, you may want to think about what you will do if the tests show you have a high chance of getting osteoporosis. For more information, see:

Osteoporosis: Should I Have a Dual-Energy X-Ray Absorptiometry (DEXA) Test?