The science and practice of testing for Huntington's disease
What are the possible test results?
Test results are generally disclosed in person by a genetic counselor. The participant has a right to decide before the delivery of results to not receive this information. Individuals who do return to the testing center receive one of three possible test results:
A NEGATIVE test result:
If the CAG repeat size is found to be in the normal range (less than 35 repeats), then the individual does not carry the genetic mutation for Huntington’s disease and is not at risk for developing the disorder.
A POSITIVE test result:
If the Huntington gene contains 40 or more CAG repeats, then the individual carries the HD allele and will eventually develop Huntington’s disease. A positive test result provides little, if any, information about what to expect in terms of the age of onset, the rate of progression, or the severity of symptoms of HD. Although there is a statistical association between a greater number of CAG repeats and earlier age at onset, variability in this relationship can prevent repeat length from accurately predicting age at onset.
An UNINFORMATIVE test result:
If the CAG repeat length is in the intermediate range (36-39 repeats), it is unclear whether or not the individual will develop HD. Only 1-4% of people tested for HD receive this inconclusive result. Further research may provide more information about the significance of this intermediate number of CAG repeats.
Receiving any one of these three test results carries intense clinical and psychological implications. Thus, genetic testing is generally followed by regularly scheduled post-test counseling sessions.
Last Modified: 9-13-02
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