One proposal for determining the appropriate use of predictive tests is the "rule of earliest onset." Simply put, the rule states that "genetic testing should be permitted no earlier than the age of first possible onset of disease" (Kodish, p. 391). This guideline allows for the possibility that medical benefit may outweigh potential harms. Employing this basic rule provides several advantages. First, predictive testing is limited to those children for whom there is a potential medical benefit. Though this does not eliminate the possibility that decisions to test will be fueled by additional motivations, it ensures that benefit to the child will be present. Secondly, by delaying testing until an age when symptoms may occur, one maximizes the likelihood that the now older child can participate in the decision-making process. Finally, it is a family-specific guideline for testing that accounts for variation in the age of onset. For example, even though the majority of Huntington's disease occurs in adults, approximately 10 percent of cases are juvenile. In these families, the disease is typically transmitted through a father whose own disease had an earlier than expected presentation. If predictive testing for a child is being considered, and the history reveals that in this particular family the father is the affected individual and his symptoms developed in his twenties, then the rule of earliest onset for this family would suggest testing an adolescent.
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