The discovery of the HD gene in 1993 resulted in a direct genetic test to make or confirm a diagnosis of HD in an individual who is exhibiting HD-like symptoms. Using a blood sample, the genetic test analyzes DNA for the HD mutation by counting the number of repeats in the HD gene region. Individuals who do not have HD usually have 28 or fewer CAG repeats. Individuals with HD usually have 40 or more repeats. A small percentage of individuals, however, have a number of repeats that fall within a borderline region.

Diagnosis by CAG repeats is loosely related to the age of onset and not related to the duration (severity) of HD. Onset and duration can be controlled by the interventions of diet, exercise and spirit as demonstrated in HD mice models. The proactive assumption is that these results apply to humans unless proven otherwise.

How is HD Diagnosed?
Huntington's Disease: Hope Through Research
National Institute of Neurological Disorders and Stroke
(NINDS 1998)

CAG v Onset
A Physician's Guide to the Management of Huntington's Disease 2nd ed.