A bone density scan compares your bone density with the bone density expected for a young healthy adult or a healthy adult of your own age, gender and ethnicity.

The difference is calculated as a standard deviation SD score. This measures the difference between your bone density and the expected value. The difference between your measurement and that of a young healthy adult is known as a T score,.

The difference between your measurement and that of someone of the same age is known as a Z score. If your Z score is below -2, your bone density is lower than it should be for someone of your age.

Z scores are usually used for children and people under 30 who are still growing. Although BMD results provide a good indication of bone strength, the results of a bone density scan will not necessarily predict whether you'll get a fracture. For example, someone with low bone density may never break a bone, whereas someone with average bone density may have several fractures.

This is because other factors, such as age, sex or whether you have previously had a fall, also determine if you're likely to sustain a fracture. Contrary to popular belief, healthy bones are not solid.

Their interiors are made of a honeycomb structure with tiny holes to keep them light and springy. However, bones that have lost a lot of mineral density have much larger holes as well as thin outer walls, which can increase their risk of breaking.

So bone density is important. Age and lifestyle can lower bone density and increase risk for conditions such as osteoporosis dangerously low bone density and osteopenia, which is not as extreme as osteoporosis.

Low bone density is a widespread problem: Each year an estimated 1. At Yale Medicine, our endocrinologists specialize in metabolic bone disease. These are conditions caused by deficiencies in minerals such as calcium, phosphorus, magnesium or vitamin D or problems with the hormones that regulate our blood minerals.

Sometimes metabolic bone disease can be due to problems in the proteins and chemicals that make up the bones themselves. Low bone density can lead to serious medical conditions that could result in bone fractures.

The most worrisome condition is osteoporosis, where low bone density causes holes inside the bone to widen and the outer walls of the bone the cortex to thin. This causes the bone to be more fragile.

People with osteoporosis are at a much greater risk for fracture with little or no trauma. For example, an older person can get a hip fracture as a result of a simple fall from a standing position. Hip fractures are the most serious of all osteoporotic fractures and can cause loss of mobility and independence and even death.

Osteopenia is similar to osteoporosis but where the bone density is not as low. The Vertebral Fracture Assessment VFA , a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine, may be recommended for older patients, especially if:.

On the day of the exam you may eat normally. You should wear loose, comfortable clothing, avoiding garments that have zippers, belts or buttons made of metal.

Objects such as keys or wallets that would be in the area being scanned should be removed. Remove jewelry, removable dental appliances, eyeglasses, and any metal objects or clothing that might interfere with the x-ray images.

Inform your physician if you recently had a barium examination or have been injected with a contrast material for a computed tomography CT scan or radioisotope scan.

You may have to wait 10 to 14 days before undergoing a DXA test. Women should always tell their doctor and technologist if they are pregnant.

Doctors will not perform many tests during pregnancy to avoid exposing the fetus to radiation. If an x-ray is necessary, the doctor will take precautions to minimize radiation exposure to the baby. See the Radiation Safety page for more information about pregnancy and x-rays.

Most of the devices used for DXA are central devices, which are used to measure bone density in the hip and spine. They are usually located in hospitals and medical offices. Central devices have a large, flat table and an "arm" suspended overhead. Peripheral devices measure bone density in the wrist, heel or finger and are often available in drugstores and on mobile health vans in the community.

The pDXA devices are smaller than the central DXA devices, weighing only about 60 pounds. They may have a portable box-like structure with a space for the foot or forearm to be placed for imaging.

Other portable technologies such as specially designed ultrasound machines, are also sometimes used for screening. However, central DXA is the standard technique. The DXA machine sends a thin, invisible beam of low-dose x-rays with two distinct energy peaks through the bones being examined.

One peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is a patient's bone mineral density. DXA machines feature special software that compute and display the bone density measurements on a computer monitor.

In the central DXA examination, which measures bone density of the hip and spine, the patient lies on a padded table. An x-ray generator is located below the patient and an imaging device, or detector, is positioned above.

To assess the spine, the patient's legs are supported on a padded box to flatten the pelvis and lower lumbar spine. To assess the hip, the patient's foot is placed in a brace that rotates the hip inward. In both cases, the detector is slowly passed over the area, generating images on a computer monitor.

You must hold very still and may need to hold your breath for a few seconds while the technologist takes the x-ray. This helps reduce the possibility of a blurred image. The technologist will walk behind a wall or into the next room to activate the x-ray machine. The peripheral tests are simpler.

The finger, hand, forearm or foot is placed in a small device that obtains a bone density reading within a few minutes.

An additional procedure called Vertebral Fracture Assessment VFA is now being done at many centers. VFA is a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine.

The DXA bone density test is usually completed within 10 to 30 minutes, depending on the equipment used and the parts of the body being examined. You will probably be asked to fill out a questionnaire that will help the doctor determine if you have medical conditions or take certain medications that either increase or decrease your risk of a fracture.

The World Health Organization has recently released an online survey that combines the DXA results and a few basic questions and can be used to predict year risk of hip fracture or other major osteoporotic fractures for post-menopausal women. Routine evaluations every two years may be needed to see a significant change in bone mineral density, decrease or increase.

Few patients, such as patients on high dose steroid medication, may need follow-up at six months. A radiologist , a doctor trained to supervise and interpret radiology examinations, will analyze the images. The radiologist will send a signed report to your primary care or referring physician who will discuss the results with you.

DXA scans are also interpreted by other physicians such as rheumatologists and endocrinologists. A clinician should review your DXA scan while assessing the presence of clinical risk factors such as:. T score — This number shows the amount of bone you have compared with a young adult of the same gender with peak bone mass.

A score of -1 and above is considered normal. A score between A score of The T score is used to estimate your risk of developing a fracture and also to determine if treatment is required.

Z score — This number reflects the amount of bone you have compared with other people in your age group and of the same size and gender. If this score is unusually high or low, it may indicate a need for further medical tests. Small changes may normally be observed between scans due to differences in positioning and usually are not significant.

Doctors take special care during x-ray exams to use the lowest radiation dose possible while producing the best images for evaluation.