do all gene carriers develop disease?

  I know that the gene for HD was identified in 1993 and that direct testing has been available since then.I am also aware that the disease is completely dominant.If the test shows CAG repeats of more than 40 (30-40 being a known grey area)the patient is said to have a 96% risk of developing the disease.Can you tell me what data is available on the number of people diagnosed with this level of risk who have subsequently developed the disease? Is it possible that the principle that "all mackerel are fish,but not all fish are mackerel" could apply?
  Also,could you tell me what are the rates for false positive test results?
  I appreciate that different test centers may have different rates,but I would like to know an average figure.I am aware that counting CCG repeats can cause the result to be falsely high,but excluding this,what is the overall accuracy of the tests?
  Many thanks,
  Sarah
=
I'm not certain exactly what you are asking. The test is considered to be pretty reliable, but it must be interpreted in context. That is, if you do the HD test in a person who has absolutely no family history, I suppose that the false positive rate would be higher than it would be for individuals with demonstrated HD in the family who are tested with more than 37-40 repeats.
If the risk is quoted as "96%" then I would suppose that the data support that. A MedLine literature review (at the library of your local medical school) should reveal the original papers with this data.
The answer to any question about genetic disease is strongly influenced by the personal and family history of the individual asking the question. That is why your question is so hard to answer. If you have been tested, it is important for you to discuss this with your doctor. If you would like a second opinion, we do have specialists at CCF who diagnose and treat HD, and you may call 800 223-2273 and ask for neurology appointments. CCF MD mdf.