What is Predictive Testing
Adapted from: Predictive Testing for Huntington's Disease, by the Huntington's Society of Canada and Guidelines for Genetic Testing for Huntington's Disease, (revised 1994) by the Huntington's Disease Society of America.
The identification of the HD gene allows individuals who are at risk to find out now whether or not they have inherited the HD gene. In most instances, it may be possible to test only the person who is at risk. It is helpful to have DNA test results from an affected relative, if possible, however.
The following example illustrates how predictive testing can be done in a family. Mary has requested predictive testing. Her father, John, has Huntington's disease. A sample of her father's DNA is analyzed and it is determined that he has 15 repeats on one number four chromosome and 45 repeats on the other. The fact that he has a gene with more than 40 repeats supports the clinical diagnosis of HD. Mary's DNA can then be assessed. If she has received 10 repeats and 45 repeats, the 10 must have come from her mother, Angie, and the 45 repeats must have been inherited from her father. This indicates that she has inherited the HD gene, and she will develop HD. If, on the other hand, she has inherited 10 repeats and 15 repeats, she received the gene containing the 15 repeats from her father. This is not the gene causing HD, and therefore she will not develop HD.
Predictive testing provides a highly accurate answer as to whether or not Mary has inherited the gene causing HD. It does not, however, say anything about her current health with respect to Huntington's disease or when she might begin to experience symptoms.
In a small number of families, predictive testing may not be possible. This may occur if the clinical diagnosis in the family is HD, but none of the affected individuals in the family demonstrate the increased number of repeats found in the HD gene. This could happen because the family has been misdiagnosed and the disorder is not HD, or potentially because there is another change in this gene causing HD that we do not yet know how to detect. Another reason why testing could be uninformative is that the at-risk person may inherit a gene that has an "intermediate" repeat size between 36 and 39. Such an intermediate, or equivocal, result occurs infrequently.