Norm of Bone Densitometry (Dual-Energy X-Ray Absorptiometry, DEXA)
|Bone mineral density (BMD)||Percentage within range of standard deviations from the norm provided by the manufacturer of the device used for the test. Each manufacturer provides a database of BMDs from many persons for comparison to those with similar age, race, sex, and ethnicity. Low BMD indicates androgen deprivation in prostate cancer, cystic fibrosis, or osteoporosis.|
|Z-score||(Percent young adult) within range of manufacturer's norms.|
|T-score||(Age-matched) within range of manufacturer's norms.|
Usage of Bone Densitometry (Dual-Energy X-Ray Absorptiometry, DEXA)
Measurement of bone mass or density, evaluation for predicting risk of fracture, skeletal morphometry, and body-composition analysis (less common). Bone densitometry is one of the diagnostic criteria for osteoporosis established by the World Health Organization.
Description of Bone Densitometry (Dual-Energy X-Ray Absorptiometry, DEXA)
Considered the standard of comparison often used for diagnosis of osteoporosis, bone densitometry, approved by the FDA in 1988, is also known as dual-energy x-ray absorptiometry (DEXA). The procedure is carried out using an x-ray device that scans the heel, finger, lumbar spine, and nondominant proximal femur or forearm and determines bone mineral density (BMD). The site selected for scanning is determined by the purpose of the test. The BMD measurement at one site can allow prediction of the risk of fracture at another site of the body. For assessment of general fracture risk, either the spine or the neck of the femur is measured. This test is not indicated if treatment decisions will not be affected by the results. In addition to the BMD, results are also often reported as a “Z-score” and a “T-score.” The Z-score represents a comparison to the peak bone mass scores in other persons and helps one to predict the risk of future fracture. The T-score represents a comparison to the person's bone density to that of others of the same age.
Professional Considerations of Bone Densitometry (Dual-Energy X-Ray Absorptiometry, DEXA)
Consent form NOT required.
During pregnancy, risks of cumulative radiation exposure to the fetus from this and other previous or future imaging studies must be weighed against the benefits of the procedure. Although formal limits for client exposure are relative to this risk-benefit comparison, the United States Nuclear Regulatory Commission requires that the cumulative dose equivalent to an embryo/fetus from occupational exposure not exceed 0.5 rem (5 mSv). Radiation dose to the fetus is proportional to the distance of the anatomy studied from the abdomen and decreases as pregnancy progresses. For pregnant clients, consult the radiologist/radiology department to obtain estimated fetal radiation exposure from this procedure.
- Notify radiologist if client is pregnant.
- Remove any metal items, such as buckles, jewelry, or buttons, from the area to be scanned.
- The client is positioned for the radiograph. For spinal or femoral densitometry, the client lies on a radiograph table. For peripheral densitometry, the client may sit upright in a chair and place the foot, finger, or forearm on a smaller device designed for peripheral densitometry.
- The scan is taken.
Client and Family Teaching
- The procedure is painless and uses minimal radiation.
- The scan takes only a few minutes.
- It is important to lie as still as possible during the scan.
Factors That Affect Results
- The trabecular area of the spine is the preferred site for repeated testing because it has a high rate of bone turnover and thus will show the greatest magnitude of change with treatment for osteoporosis.
- Spinal abnormalities such as scoliosis can impair accuracy of results. An additional method of evaluation of bone density should be used in these persons.
- Other techniques used to measure bone density include the following: single x-ray absorptiometry (SXA), quantitative computed tomography (QCT), and quantitative ultrasonography (QUS).
- See also Bone ultrasonometry.