A “positive” predictive test result means that the individual has inherited the DNA change associated with HD:
A positive genetic test result means that the person will have symptoms of HD at some point in their life.
- The number of CAG repeats on one gene is 36 or greater. Except in extremely rare cases, the other gene has a normal number of repeats with 26 or less.
- The accuracy of this result is close to 100%, indicating that the person has the DNA change associated with the eventual clinical presentation of HD.
- A positive genetic test result does not tell:
- whether a person has physical signs of HD or
- at exactly what age a person will start to have symptoms or
- what those symptoms will be.
The number of CAG repeats in the HD gene has been shown to be related to age of disease onset. However, the age an individual will experience symptom onset cannot be accurately predicted by this relationship. There is a great deal of variability in age of onset between individuals with identical CAG sizes, even for individuals within the same family.
For more information please see tips for the gene positive.