Predictive testing of 25 percent at-risk individuals for Huntington disease (1987-1997)

Before the mutation causing Huntington disease was identified, predictive t esting of 25% at-risk persons with a 50% at-risk parent who did not wish to know his/her genetic status, was only possible by exclusion testing. The e xclusion test, using linked markers, ensures the parent's wish not to know because the parent's risk is not changed, When mutation analysis became ava ilable in 1993, new testing options for 25% at-risk persons emerged: viz., the exclusion-definitive test and direct mutation analysis. These new tests not only disclose the risk of the test candidate, but may also change the risk of the at-risk parent and siblings. The testing options for 25% at-ris k test applicants and their consequences are discussed and the testing proc edures and results of testing 64 25% at-risk persons in the period 1987 to 1997 are described. Relatives received unsought information in 56% of the t est procedures before and 34% after the mutation was identified. A decision tree and guidelines for predictive testing of 25% at-risk test applicants are proposed. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88: 662-668,1999. (C) 1999 Wiley-Liss, Inc.