United Kingdom experience with presymptomatic testing of individuals at 25% risk for Huntington's disease.

Between 1994 and 1998, the 23 UK genetics departments, which form the UK Huntington's Disease Consortium, have undertaken 161 direct mutation adult predictive tests on individuals whose at-risk parent was alive or had died without showing signs of Huntington's disease (HD) (5.7% of total UK tests). This study describes the number of requests for 25% risk predictive testing for HD in 1994 (the first year in which direct testing was available in the UK), and also a descriptive survey of the first 85 tests. In total, 85 tests were performed in the first 2 years of direct mutation testing, ten (11.8%) tests were mutation positive, 73 (85.91%) were negative and two (2.3%) were equivocal. The at-risk parent was alive in 54 (63.5%) cases. Four of the ten mutation positive candidates had parents who were alive and therefore received a prediction through their child. All centres included a discussion about the effect of testing on the 50% at-risk parent and all offered to see the parent for counselling. Of the 87 applicants for testing during 1994, 31 (35.6%) withdrew, this is higher than the 25% withdrawal rate for the 50% risk candidates. The candidates who withdrew were significantly younger and had more parents who were alive than those who continued with testing. Seven of the 31 candidates who withdrew from testing had at-risk parents who decided to be tested in the first instance. During the counselling process, issues were raised relating to pre-test agreements and family secrecy. This study indicates the importance of pre-test counselling and the involvement of the parent in the counselling process. 25% risk testing for HD is now being offered in the majority of UK centres. As more genes are identified for late-onset conditions, it is important that the complexities of 25% risk testing for late-onset conditions are not underestimated. This limited survey does not investigate how individual families cope with the psychological and social issues raised by this study and further research in this area is needed.