Osteoporosis - Limitations of DEXA Technology
Several limitations of DEXA scanning should be noted. Criteria used to define “normal population” for the reference databases differ among instruments so the same measured value may lie within different parts of the reference range and may be assigned a different fracture risk depending on the system used. However, differences in normative data between manufacturers (with variations of as much as an entire standard deviation) are being corrected by the information collected in the NHANES II. As a result, experts recommend that longitudinal follow-up be performed using the same instrument, whenever possible.
Quality control is also important to maintain high precision in BMD measurements. Spinal BMD can be affected by the presence of scoliosis and vertebral compression fractures. Single vertebrae are difficult to define technically when compression fractures have occurred. Also, the distribution of osteoporosis could vary within the spine, with differential involvement of the dorsal spine and lumbar spine. As a result, BMD values do not discriminate between patients with and those without fractures. Frequency of densitometry testing is limited, because clinically meaningful changes depend on adequate time periods between measurements. In such cases, treatment responses may be monitored with measurements of biochemical markers.
Access to BMD Measurement Lack of access to BMD testing may limit its usefulness in areas where the equipment is not yet available, such as in smaller rural communities and for women who have transportation difficulties (e.g., frail elderly women). On July 1, 1998, Medicare announced coverage for all Food and Drug Administration (FDA)-approved bone density technologies. As a result, access to DEXA testing should improve and more patients should be able to undergo BMD assessment for determination of fracture risk.
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