Dual-energy x-ray absorptiometry is a painless, noninvasive imaging test that measures a person's bone mineral density. More commonly known as a DEXA scan, a bone density test or bone densitometry, it is considered the most accurate tool available to diagnose and monitor osteoporosis.

Most bone densitometry tests send an x-ray beam through the bones. The thin beam contains much less radiation than standard x-rays. The beam has two energy peaks. Soft tissue absorbs one of these peaks, and bone absorbs the other. The amount of radiation absorbed in the tissue is subtracted from the total absorbed, and the remaining amount indicates the patient's bone density. Some bone density tests involve ultrasound instead of x-rays.

The two major forms of bone densitometry are distinguished by the type of equipment used and the area of the body that is imaged:

• DEXA (or DXA) bone densitometry. This procedure is performed using large equipment known as a central device. It is the more commonly performed type of densitometry and focuses on the lower spine and hips, where fractures associated with osteoporosis most often occur.
  
  
• pDEXA bone densitometry. This procedure involves a portable machine known as a peripheral device. It measures bone loss in the fingers, heels or wrists. Because of the portability, it is getting more popular.
  
  

Patients are urged to closely follow all preparatory recommendations made by their physician. In most cases, such preparation is minimal. The test itself will unfold differently depending on which procedure take place. When a central device is used, the patient lies down on a padded table for a few minutes while a mechanical arm-like device with an imager passes over the body. When a peripheral device is used, the patient inserts an arm or foot into the device while an image is captured.

After testing is complete, the information obtained during the test is displayed on a computer monitor. A physician known as a radiologist examines the image and makes a diagnosis that is expressed in the form of two scores:

• T score. A number that indicates the density of bone the patient has compared to someone of the same sex who has peak bone mass (usually a healthy 30-year-old adult).
  
  
• Z score. A number that indicates the density of bone the patient has compared to someone in the same age group, body size and sex.
  
  

After a diagnosis has been suggested, the information is forwarded to the patient's primary care physician. This physician will then discuss the results with the patient and develop a treatment plan. The test involves only a minimal exposure to radiation and has no significant risks.