Bone Densitometry or Bone Density 

What is a bone density (DEXA) test and how is it used to diagnose?

A bone mineral density test determines if you have osteoporosis, a disorder characterized by bones that are more fragile and more likely to break.

The test uses X-rays to measure how many grams of calcium and other bone minerals are packed into a segment of bone. The bones that are most commonly tested are in the spine, hip and sometimes the forearm.

Doctors use bone density testing to:

• Identify decreases in bone density before you break a bone
• Determine your risk of broken bones (fractures)
• Confirm a diagnosis of osteoporosis
• Monitor osteoporosis treatment

The higher your bone mineral content, the denser your bones are. And the denser your bones, the stronger they generally are and the less likely they are to break.

The United States Preventative Task Force (USPSTF) recommends women 65 or older have routine tests to measure bone thickness. If you are at an increased risk for fracture cause by osteoporosis, routine screening should begin sooner versus later.

Although osteoporosis is more common in older women, men also can develop the condition. Regardless of your sex or age, your doctor may recommend a bone density test if you've:

• Lost height. People who have lost at least 1.5 inches (3.8 centimeters) in height may have compression fractures in their spines, for which osteoporosis is one of the main causes.
• Fractured a bone. Fragility fractures occur when a bone becomes so fragile that it breaks much more easily than expected. Fragility fractures can sometimes be caused by a strong cough or sneeze.
• Taken certain drugs. Long-term use of steroid medications, such as prednisone, interferes with the bone-rebuilding process which can lead to osteoporosis.
• Had a drop in hormone levels. In addition to the natural drop in hormones that occurs after menopause, women's estrogen may also drop during certain cancer treatments. Some treatments for prostate cancer reduce testosterone levels in men. Lowered sex hormone levels weaken bone.
  
  

Limitations of bone density testing include:

• Differences in testing methods. Devices that measure density of the bones in the spine and hip are more accurate but cost more than do devices that measure density of the peripheral bones of the forearm, finger or heel.
• Previous spinal problems. Test results may not be accurate in people who have structural abnormalities in their spines, such as severe arthritis, previous spinal surgeries or scoliosis.
• Radiation exposure. Bone density testing uses X-rays, but the amount of radiation exposure is usually very small. Even so, pregnant women should discuss the risks with their doctor prior to having the test.
• Lack of information about the cause. A bone density test can confirm that you have low bone density, but it can't tell you why. To answer that question, you need a more complete medical evaluation.
• Limited insurance coverage. Not all health insurance plans pay for bone density tests, so ask your insurance provider beforehand if this test is covered.

A small, portable machine can measure bone density in the bones at the far ends of your skeleton, such as those in your finger, wrist or heel. The instruments used for these tests are called peripheral devices and are often used at health fairs.

Because bone density can vary from one location in your body to another, a measurement taken at your heel usually isn't as accurate a predictor of fracture risk as a measurement taken at your spine or hip. Consequently, if your test on a peripheral device is positive, your doctor might recommend a follow-up scan at your spine or hip to confirm your diagnosis.

What to Expect

Before Arriving

Be sure to tell your doctor beforehand if you've recently had a barium exam or had contrast material injected for a CT scan or nuclear medicine test. Contrast materials might interfere with your bone density test.

Food and medications

Avoid taking calcium supplements for at least 24 hours before your bone density test.

Clothing and personal items

Wear loose, comfortable clothing and avoid wearing clothes with zippers, belts or buttons.

Leave your jewelry at home and remove all metal objects from your pockets, such as keys, money clips or change.

At some facilities, you may be asked to change into an examination gown.

During the exam

During the scan, a large scanning arm will be passed over your body to measure bone density in the center of the skeleton.

As the scanning arm is moved slowly over your body, a narrow beam of low-dose 

X-rays will be passed through the part of your body being examined.

This will usually be your hip and lower spine to check for weak bones (osteoporosis).

But as bone density varies in different parts of the skeleton, more than one part of your body may be scanned.

Some of the X-rays that are passed through your body will be absorbed by tissue, such as fat and bone.

An X-ray detector inside the scanning arm measures the amount of X-rays that have passed through your body.

This information will be used to produce an image of the scanned area.

The scan usually takes 10 to 20 minutes. You'll be able to go home after you have had it done.

Results

Your bone density test results are reported in two numbers: T-score and Z-score.

T-score

Your T-score is your bone density compared with what is normally expected in a healthy young adult of your sex. Your T-score is the number of units — called standard deviations — that your bone density is above or below the average.