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Bone Mineral Densitometry
Dual-energy x-ray absorptiometry (DEXA, DXA): Two x-rays of different energy levels measure bone mineral density and predict risk of fracture.
Single-energy x-ray absorptiometry (SXA): A single-energy x-ray measures bone density at peripheral sites.
Quantitative computed tomography (QCT): QCT is used to examine the lumbar vertebrae. It measures trabecular and cortical bone density. Results are compared to a known standard. This test is the most expensive and involves the highest radiation dose of all techniques.
Radiographic absorptiometry (RA): A standard x-ray of the hand. Results are compared to a known standard.
Ultrasound densitometry: Studies bone mineral content in peripheral densitometry sites such as the heel or wrist. It is not as precise as x-ray techniques but is less expensive than other techniques.
Area of applicationLumbar spine, heel, hip, wrist, whole body.
Osteoporosis is a condition characterized by low BMD, which results in increased risk of fracture.The National Osteoporosis Foundation estimates that 4 to 6 million postmenopausal women in the United States have osteoporosis, and an additional 13 to 17 million (30% to 50%) have low bone density at the hip. It is estimated that one of every two women will experience a fracture as a result of low bone mineral content in her lifetime.The measurement of BMD gives the best indication of risk for a fracture. The lower the BMD, the greater is the risk of fracture.The most common fractures are those of the hip, vertebrae, and distal forearm. Bone mineral loss is a disease of the entire skeleton and is not restricted to the areas listed.The effect of the fractures has a wide range, from complete recovery to chronic pain, disability, and possible death.
The BMD values measured by the various techniques cannot be directly compared. Therefore, they are stated in terms of standard deviation (SD) units.The patient’s T-score is the number of SD units above or below the average BMD in young adults. A Z-score is the number of SD units above or below the average value for a person of the same age as the measured patient. Since bone loss occurs naturally as part of the aging process, using a patient’s Z-score in comparison to a person of the same age could be misleading, especially in the early development of osteoporosis.The World Health Organization has defined normal bone density as being within (above or below) 1 SD of the mean for young adults. Low bone density is defined as density below 1 SD and 2.5 SD the mean for young adults, bone density 2.5 SD or more below the mean for young adults is indicative of osteoporosis (osteopenia), and bone density more than 2.5 SD below the mean for young adults is defined as severe (established) osteoporosis. The baseline age for young adults is approximately 30 years of age. For most BMD readings, 1 SD is equivalent to 10% to 12% of the average young-normal BMD value. A T-score of −2.5 is therefore equivalent to a bone mineral loss of 30% when compared to a young adult.
This procedure is contraindicated for
- Patients who are pregnant or suspected of being pregnant, unless the potential benefits of a procedure using radiation far outweigh the risk of radiation exposure to the fetus and mother.
- Determine the mineral content of bone
- Determine a possible cause of amenorrhea
- Establish a diagnosis of osteoporosis
- Estimate the actual fracture risk compared to young adults
- Evaluate bone demineralization associated with chronic renal failure
- Evaluate bone demineralization associated with immobilization
- Monitor changes in BMD due to medical problems or therapeutic intervention
- Predict future fracture risk
- Normal bone mass with T-score value not less than −1.
Abnormal findings related to
- Osteoporosis is defined as T-score value less than −2.5.
- Low bone mass or osteopenia has T-scores from −1 to −2.5.
- Fracture risk increases as BMD declines from young-normal levels (low T-scores).
- Low Z-scores in older adults can be misleading because low BMD is very common.
- Z-scores estimate fracture risk compared to others of the same age (versus young-normal adults).
- BMD test results may be lower in individuals receiving corticosteroid therapy; ideally BMD testing should be performed before a patient is placed on a regimen of chronic steroid therapy to obtain a valid baseline.
Factors that may impair clear imaging
- Inability of the patient to cooperate or remain still during the procedure because of age, significant pain, or mental status.
- Metallic objects within the examination field (e.g., jewelry, earrings, and/or dental amalgams), which may inhibit organ visualization and can produce unclear images.
- The use of anticonvulsant drugs, cytotoxic drugs, tamoxifen, glucocorticoids, lithium, or heparin, as well as increased alcohol intake, increased aluminum levels, excessive thyroxin, renal dialysis, or smoking, may affect the test results by either increasing or decreasing the bone mineral content.
- Consultation with a health-care provider (HCP) should occur before the procedure for radiation safety concerns regarding younger patients or patients who are lactating. Pediatric & Geriatric Imaging Children and geriatric patients are at risk for receiving a higher radiation dose than necessary if settings are not adjusted for their small size. Pediatric Imaging Information on the Image Gently Campaign can be found at the Alliance for Radiation Safety in Pediatric Imaging (www.pedrad.org/associations/5364/ig/).
- Risks associated with radiation overexposure can result from frequent x-ray or radionuclide procedures. Personnel in the room with the patient should stand behind a shield, or leave the area while the examination is being done. Personnel working in the examination area should wear badges to record their radiation exposure level.
- Vertebral fractures may cause complications including back pain, height loss, and kyphosis.
- Limited activity, including difficulty bending and reaching, may result.
- Patient may have poor self-esteem resulting from the cosmetic effects of kyphosis.
- Potential restricted lung function may result from fractures.
- Fractures may alter abdominal anatomy, resulting in constipation, pain, distention, and diminished appetite.
- Potential for a restricted lifestyle may result in depression and other psychological symptoms.
- Possible increased dependency on family for basic care may occur.
Other considerations as a result of altered BMD, not the BMD testing process
Nursing Implications and Procedure
- Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
- Patient Teaching: Inform the patient this procedure can assist in assessing bone density.
- Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex, iodine, seafood, contrast medium, anesthetics, or dyes.
- Obtain a history of the patient’s musculoskeletal system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures
- Note any recent procedures that can interfere with test results, including examinations using iodine-based contrast medium.
- Record the date of the last menstrual period and determine the possibility of pregnancy in perimenopausal women.
- Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
- Review the procedure with the patient. Address concerns about pain related to the procedure and explain that some pain may be experienced during the test, or there may be moments of discomfort. Inform the patient that the procedure is usually performed in a radiology department by a HCP, and staff, specializing in this procedure and takes approximately 60 min.
- Instruct the patient to remove jewelry and other metallic objects from the area to be examined.
- Note that there are no food, fluid, or medication restrictions unless by medical direction.
- Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
- Potential complications: N/A
- Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
- Ensure that the patient has removed all external metallic objects from the area to be examined prior to the procedure.
- Instruct the patient to void prior to the procedure and to change into the gown, robe, and foot coverings provided. Patient’s clothing may not need to be removed unless it contains metal that would interfere with the test.
- Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
- Instruct the patient to cooperate fully and to follow directions. Instruct the patient to remain still throughout the procedure because movement produces unreliable results.
- Place the patient in a supine position on a flat table with foam wedges, which help maintain position and immobilization.
- Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
- Recognize anxiety related to test results, and be supportive of perceived loss of independent function. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Provide contact information, if desired, for the National Osteoporosis Foundation (www.nof.org).
- Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
- Depending on the results of this procedure, additional testing may be needed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms, previous BMD values, and other tests performed.
- Related tests include ALP, antibodies anticyclic citrullinated peptide, ANA, arthrogram, arthroscopy, biopsy bone, bone scan, calcium, CRP, collagen cross-linked telopeptides, CT pelvis, CT spine, ESR, MRI musculoskeletal, MRI pelvis, osteocalcin, PTH, phosphorus, radiography bone, RF, synovial fluid analysis, and vitamin D.
- Refer to the Musculoskeletal System table at the end of the book for related tests by body system.