Dual X-ray Densitometry (DXA), the most advanced and reliable method to screen for osteoporosis quickly and painlessly, is available in the office.
DXA scans are very accurate in assessing a patient's bone fracture risk and need for treatment. The DXA scan is more sensitive than ordinary X-rays in diagnosing bone loss at an early stage. Safe and painless, the 15-minute DXA scan is the most reliable test to determine even the earliest stages of bone loss associated with osteoporosis. It is also used in determining a person's risk for hip fractures. The DXA scan is like an X-ray, except it uses less radiation—only about 20 percent of a standard chest X-ray.
Osteoporosis is a condition of low bone mineral density that causes loss of bone strength. This can lead to deformities, bone pain and fractures. Until recently, doctors couldn't diagnosis patients with osteoporosis until a bone fracture occurred. DXA scanning has emerged as the preferred imaging technique to diagnose and monitor osteoporosis. It is also used for scanning for early stages of low bone density, even before symptoms occur. In performing a bone mineral density assessment, DXA:
With the information from the DXA scan, the doctor can start and monitor treatment if needed.
There are a number of other ways for calculating bone density or bone turnover. These are generally not as satisfactory as DXA scanning. DXA scanning is often performed in an office or outpatient facility. The procedure does not require the patient to be in any kind of “tube.” There are no injections, contrast dyes or medications of any sort necessary to do the examination. The amount of radiation is very low, so people with pacemakers, artificial joints or metal inside the body can safely have a DXA scan.
How does DXA work?
With the patient lying on his or her back, a small arm passes over the body, about two feet above the stomach and hips, scanning the bone density at the hips and spine. (The hips and spine are measured because that is where osteoporotic fractures occur most often.) A technologist sits next to the examination table to monitor the procedure from a computer, and a medical radiologist reads the results.
How is bone density measured?
The table below compares the T score or Z score to an equivalent percentile. For example, if the patient was pre-menopausal, and the T score was 0.0, half of the pre-menopausal population has greater bone density, and half less. If the score were -1.0, there would be 84% with greater bone density.
Fortunately, there are tables that allow an easy conversion of standard deviations to percentages. For example, a Z-Score of -1.8 means that patients at the 4th percentile, or 96% of women in that category, have greater bone density than the patient. A T-Score of -2.0 means a patient at the 2nd percentile, or 98% of pre-menopausal women have greater bone density than the patient. A T-score between -1 and -2 represents significant bone loss or osteopenia. The World Health Organization considers a T-Score of -2.5 or below to signify osteoporosis. And many U.S. organizations deem -2.0 or below to represent osteoporosis.
NOTE: Lead aprons or other protective garments are unnecessary, as the amount of radiation is so low that the garments are deemed excessive. As with any medical procedure, if you are pregnant or trying to become pregnant, you should not have this scan.
WHAT IS A BONE MINERAL DENSITY (BMD) TEST?
RISK FACTORS FOR OSTEOPOROSIS
WHY WOULD I NEED A BMD TEST?
HOW IS BONE DENSITY MEASURED?
HOW DO I PREPARE FOR AN EXAM?
WHAT CAN I EXPECT DURING MY EXAM?
WHAT DO THE RESULTS MEAN?
DXA (Dual Energy X-ray Absorptiometry) is the most widely available method of bone densitometry, and most insurance plans will cover the cost for the test, given that certain medical indicators are present. Bone mineral density measurement with DXA is painless, and requires no injections, invasive procedures, sedation, special diet or any other advance preparation. During a DXA exam, the patient lies fully clothed on a padded table while the system scans one or more areas of bone (usually the lower spine or hip). The entire exam typically takes just a few minutes to complete. While DXA uses x-rays, the radiation dose is less than during a chest x-ray. Each patient's bone density is plotted against the "normal" for a healthy young adult or against age matched control data. A radiologist or other physician then interprets the data and creates a concise report on the status of the patient's bone density.
Laboratory tests that measure the amount of collagen in urine samples can indicate bone loss. Lab tests may also be used in conjunction with DXA or other methods of bone densitometry to diagnose osteoporosis.
New methods of measuring osteoporosis using ultrasound have also been developed. One such ultrasound system measures BMD at the patient's heel and takes about a minute. The ultrasound systems for testing osteoporosis are smaller and less expensive than traditional DXA systems. These systems have recently received US Food and Drug Administration (FDA) clearance. The hope is that this more compact, lower cost system will allow this vital diagnostic test to become more widely available in the future. By primarily measuring peripheral sites such as the heel, ultrasound densitometry may not be as sensitive as techniques such as DXA or QCT that measure the spine or hip since the heel may be normal in bone density even when central sites such as the hip or spine are already significantly abnormal.
Further, density changes in the heel occur much slower than in the hip or spine. Therefore ultrasound densitometry should not be used to monitor a patient's response to therapy. However, the new ultrasound densitometry systems will allow many more people access to bone densitometry and potentially diagnose osteoporosis before a traumatic fracture occurs.
Summary of Types of Bone Mineral Density Tests
The accuracy of bone mineral density test is high, ranging from 85% to 99%. According to physicians, QCT is the most accurate BMD test and ultrasound is the least accurate of the tests. DXA is the most widely available BMD test and its accuracy is in between those of QCT and ultrasound. However, QCT is not widely available and delivers more radiation to the patient than DXA.
Patients Recommended for Bone Mineral Density Measurements:
Medicare Guidelines for Bone Densitometry:
Understanding the Results of Bone Mineral Density Tests: T-score and Z-score
To calculate a patient's T-score, physicians subtract the patient's measured BMD from a BMD reference range of women in their thirties (YN = young normal). Around age 35, bone mass is usually at its peak and fracture risk is at its lowest. This figure is then divided by the standard deviation (SD).
T Score = (BMD - YN) / SD
By determining a patient's T-score, the physician will see whether a woman has or is at risk for osteoporosis:
As a rule, a patient's fracture risk doubles for every SD (standard deviation) below the young adult reference range. For example, if a patients' T-score is -2 SD, she is at four times greater risk of a fracture than a young woman at peak bone density.
Physicians may also measure a patient's Z-score with a bone mineral density test. The Z-score compares the patient's measured BMD to the average BMD measurement for the patient's age and sex. The Z-score is not used to confirm a diagnosis of osteoporosis because a favorable BMD measurement (compared to the average BMD measurement for the patient's age group) does not mean the patient is not at risk for osteoporosis.
Bone mineral density increases until around age 35 and then levels off until menopause. During the first six to eight years of menopause, there is a sharp decline in bone mineral density. It is estimated that between 1% and 5% of bone density is lost at this time. The higher a woman's overall bone density, the less she will be affected when she loses bone density at menopause.
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