Predictive testing for Huntington disease in Victoria, Australia- 1989-2004
Introduction: This study describes the first 15 years of predictive testing for HD in Victoria, and constitutes the world’s largest study based on a single testing centre. Initially testing involved linkage analysis that gave only a modified risk of developing HD. However, the advent of direct mutation analysis in 1993 provided a definitive test that could be applied pre-symptomatically.
Method: Records held at Genetic Health Services Victoria provided data on 756 people who have sought knowledge of their genetic status in relation to HD. The data were compiled into an anonymous database and included a broad range of demographic and clinical details.
Results: 90.1% of candidates presented with a prior risk of 50%, 57.6% were female and the average age at first contact was 40.4 years (independent of gender). Of all completed tests (648), 37.5% gave a high risk or gene positive result, with another 3.2% being in the ‘grey zone’ of reduced penetrance. A non-completion rate of 14.3% was observed. This study has revealed for the first time that the length of time people have known about their at-risk status, and their level of prior exposure to severe HD both impact significantly on the timing of testing and the likelihood of it being completed. For instance, those who completed testing had known of their at-risk status for an average of 10.2 years prior to testing, compared to 5.2 years for those who withdrew. In addition, 70% of those with no significant exposure to HD chose to be tested within 1 year of learning they were at risk. Entry into the program has stabilised at around 40 per year from a peak of 87 in 1994.
Conclusion: Although a substantial majority of those who are seen in the predictive testing program receive their result, those with little knowledge or exposure to HD are significantly more likely to rush into testing and less likely to learn of their genetic status.