How is Huntington's disease diagnosed?Someone with a family history of Huntington's disease who begins to have symptoms often is diagnosed based on a physical and neurological exam. A blood test is now available to detect the presence of the genetic abnormality that causes Huntington's disease. Everyone who carries the Huntington's disease gene eventually will develop the disease,
but the test cannot predict when symptoms will begin.
A genetic test is available for confirmation of the clinical diagnosis. In this test, a small blood sample is taken, and DNA from it is analyzed to determine the CAG repeat number. A person with a repeat number of 30 or below will not develop Huntington's disease. A person with a repeat number between 35 and 40 may not develop the disease within their normal lifespan. A person with a very high number of repeats (70 or above) is likely to develop the juvenile-onset form.
Prenatal testing is available. A person at risk for Huntington's disease (a child of an affected person) may obtain fetal testing without determining whether she herself carries the gene. This "nondisclosing" test, also called a linkage test, examines the pattern of DNA near the gene in both parent and fetus, but does not analyze for the triple repeat itself. If the DNA patterns do not match, the fetus can be assumed not to have inherited the HD gene, even if present in the parent. A pattern match indicates the fetus probably has the same genetic makeup of the at-risk parent. It does not indicate whether the parent (or fetus) actually has the defective gene.