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What is the difference between a DXA and apDXA?
The DXA (or sometimes called the DEXA) and pDXA (or sometimes called pDEXA) are both used to evaluate bone density. The DXA takes up to 10 minutes to complete and the pDXA takes less than 5 minutes. The DXA measures bone density in the lumbar spine, the hip (also called proximal femur), the forearm, or the total body. The pDXA testing is usually done on the forearm at the wrist or on the heel of the foot, but it can also be used higher on the forearm or on the fingers. The pDXA machine is portable and can be used to screen bone density in large numbers of people, especially in rural areas and at health fairs. The DXA machines are not mobile and are far more expensive than the pDXA machines. DXA testing is done with the patient lying down and pDXA can be done with the patient sitting up. The DXA uses a very small amount of radiation and the pDXA even less.
The pDXA testing is only done for screening purposes. A diagnosis of osteoporosis can only be made using DXA. So if your pDXA shows some bone loss, your clinician would likely recommend a DXA to evaluate your hip and spine. pDXA testing is not considered appropriate for monitoring bone density in patients undergoing treatments for osteoporosis because response to treatments is not as evident in the bones of your hands, arms, and feet. pDXA testing on your forearm, usually your nondominant arm (for example, your left forearm if you are right handed), is not recommended if your forearm has been previously fractured, if it has a dialysis graft site, if it has been subject to prolonged immobilization, or if there is severe weakness or paralysis of that arm.
Are there other tests that are used to diagnose osteoporosis?
While other tests can be used to diagnose osteoporosis, the WHO has established the guidelines for diagnosis based only on the results of DXA testing. So most other methods for identifying osteoporosis or osteopenia are used for screening, and then if the screening test suggests reduced bone density, a DXA is ordered to make a diagnosis. The following are the other tests that are available but less widely used:
• Single energy x-ray absorptiometry (SXA), like the pDXA, is done at the wrist or the heel. Interestingly, the body part being measured for bone density is submerged in a water bath.
• Quantitative computed tomography (QCT) is a method of testing that uses CT scan technology to measure bone density in the spine. Although it provides accurate measurements, the major drawback to this test is that it exposes the patient to 50 times more radiation than the DXA test. QCT is more expensive than DXA and takes between 10 and 20 minutes to perform. It is a good option for people who have arthritis in the spine because the QCT results are not affected by changes due to arthritis that can interfere with DXA results.
• Peripheral quantitative computed tomography (pQCT) uses the same technology as the QCT but measures bone density in the forearm or wrist. This test is not routinely ordered and is used primarily in the research field.
• Radiographic absorptiometry (RA) is actually a conventional x-ray used to measure bone density in the hand (primarily the middle bones of the 2nd, 3rd, and 4th fingers). Special software and scanning equipment can measure bone density. Radiation from this test is minimal.
• Quantitative ultrasound (QUS), like other ultrasound techniques, uses sound waves instead of radiation. The heel, wrist, tibia bone in the leg, and a finger can be imaged and evaluated for bone mass.
• Radiogrammetry is a test that, like RA, uses conventional x-rays. Instead of fingers, however, two hand bones (metacarpals) are used to measure and compare bone density.
Figure 8 shows the tests that are used to measure bone density in various parts of the skeleton.
In addition to BMD tests, there are also biochemical "markers" that are measured in blood and urine to determine if specific therapies for osteoporosis are working. Biochemical marker measurements are not used for the purpose of diagnosing osteoporosis but sometimes for monitoring the progress of treatments (see Questions 39 and 69).
Figure 8 Sites in the body measured by different BMD tests. Courtesy of the National Association of Nurse Practitioners in Women's Health (NPWH).