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mdash;: Members of prepaid health plans are often considered a group for which complete histories of exposure to medically-related risk factors are known. In a population-based case-control study of radiography as a risk factor for acute myelogenous leukemia, extensive retrospective exposure data from an average of 71% of subjects' health care providers were collected. It was determined that at least 21% of prepaid health plan members received diagnostic imaging procedures outside of their plans over the 10-y period of interest and that these "outside" procedures constituted 43% of their total red bone marrow dose from all radiographic imaging. Procedures significantly more likely to have been received outside of prepaid health plans were conventional spine x rays (p = 0.004) and coronary angiograms (p < 0.0001). All "outside" coronary angiograms and 50% of "outside" spine x rays were received at hospitals. Epidemiologic studies that restrict exposure to that received within prepaid health plans may miss a substantial portion of exposure, resulting in underestimated risk estimates and a loss of statistical power.

(C) 2002 by the Health Physics Society