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A bone density test uses special X-rays to measure how many grams of calcium and other bone minerals -- collectively known as bone mineral content -- are packed into a segment of bone. The higher your mineral content, the denser your bones are. And the denser your bones, the stronger they are and the less likely they are to break. Doctors use a bone density test to determine if you have, or are at risk of, osteoporosis.[1]

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A bone mineral density (BMD) test is the best way to determine your bone health. BMD tests can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. The most widely recognized bone mineral density test is called a dual-energy x-ray absorptiometry or DXA test. It is painless - a bit like having an x ray. It can measure bone density at your hip and spine.

What Does the Test Do?
A DXA test measures your bone mineral density and compares it to that of an established norm or standard in order to give you a score. Although no bone density test is 100 percent accurate, it is the single most important predictor of whether a person will have a fracture in the future.

T-score
Most commonly, your DXA test results are compared to the ideal or peak bone mineral density of a healthy 30-year-old adult, and you are given a T-score. A score of 0 means your BMD is equal to the norm for a healthy young adult. Differences between your BMD and that of the healthy young adult norm are measured in units called standard deviations (SDs). The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.

... A T-score between +1 and -1 is considered normal or healthy. A T-score between -1 and -2.5 indicates that you have low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of -2.5 or lower indicates that you have osteoporosis. The greater the negative number, the more severe the osteoporosis.

Z-score
Sometimes, your bone mineral density is compared to that of a typical individual whose age is matched to yours. This comparison gives you a Z-score. Because low BMD is common among older adults, comparisons with the BMD of a typical individual whose age is matched to yours can be misleading. Therefore, the diagnosis of osteoporosis or low bone mass is based on your T-score. However, a Z-score can be useful for determining whether there may be an underlying disease or condition that is causing bone loss.

Low Bone Mass Versus Osteoporosis
The information provided by a BMD test can help your doctor decide which prevention or treatment options are right for you. If you have low bone mass that is not low enough to be diagnosed as osteoporosis, this is sometimes referred to as osteopenia. Low bone mass could be caused by many factors such as:
* heredity
* the development of less-than-optimal peak bone mass in your youth
* a medical condition or medication to treat such a condition that negatively affects bone
* abnormally accelerated bone loss.

While not everyone who has low bone mass will develop osteoporosis, everyone with low bone mass is at higher risk for the disease and the resulting fractures.[2]

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The reproducibility of DEXA scores is frequently reported at 1-2 percent. That 1-2 percent is the average, but the range of reproducibility can vary as much as 7 percent. Variations come from changes in machine reading (using the same machine), technologists who are doing the test, and slight changes and body positioning, all of which can affect the end results. The most frequent source of error in repeat tests is patient positioning. The technology is limited because BMD is a two-dimensional image of a three-dimensional object.[3]

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It is important for patients to get repeat BMD measurements done on the same machine each time, or at least a machine from the same manufacturer. Error between machines, or trying to convert measurements from one manufacturer's standard to another can introduce errors large enough to wipe out the sensitivity of the measurements.[4]

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Newer techniques such as ultrasound appear to offer a more cost-effective method of screening bone mass. Ultrasound measurements are usually performed at the calcaneous and it is not possible to measure sites of osteoporotic fracture such as the hip or spine. Adding an ultrasound measurement to a DEXA does not improve the prediction of fractures. Although some have said that ultrasound measures the "quality" of bone, more careful studies suggest that it mainly measures the bone mass.[5]

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Your doctor may order a bone scan to help diagnose subtle or hidden bone fractures that may not show up on a routine X-ray, such as a stress fracture. Bone scans can also help detect:
* Bone cancer
* Bone infections
* Arthritis
* Causes of unexplained bone pain

A bone scan falls under the category of nuclear medicine, which means that it uses tiny amounts of radioactive materials called tracers (radionuclides). These tracers accumulate in certain organs and tissues, such as bones. Once introduced into the body, tracers emit gamma waves of radiation, which are detected by a special camera. This camera produces images that are interpreted by radiologists or nuclear medicine specialists.[6]

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