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November 23, 2003

Testing Of Asymptomatic At-Risk Adults

Testing of asymptomatic at-risk adults for HD has been available for over ten years. It should be remembered that routine testing for an HD mutation in the presence of non-specific or equivocal symptoms is predictive testing, not diagnostic testing. When testing at-risk individuals for HD, it is helpful to test for the CAG expansion in the HD gene in an affected family member to confirm that the disorder in the family is actually HD. Predictive testing protocols usually involve pre-test interviews in which the reasons for requesting the test, the individual's knowledge of HD, the possible impact of positive and negative test results, and neurological and psychological functioning are assessed. Post-test follow-ups are also performed.

Short term follow-up of the participants in the Canadian Predictive Testing Program have revealed that predictive testing for HD may maintain or even improve the psychological well being of at-risk individuals [Wiggins et al 1992 , Bloch et al 1992] even though some had negative experiences [Morris et al 1989 , Wexler 1992 , Lawson et al 1996]. Surprisingly, about 10% of the group with decreased risk had serious difficulties adapting to their new status. The major hurdle for these individuals is the realization that they are facing an unplanned future. Overall the demand for testing of at-risk asymptomatic adults has been lower than expected in studies conducted before the availability of direct DNA testing [Craufurd et al 1989].

Data concerning the likelihood that an individual with a particular size CAG repeat will be affected by a specific age may be useful in predictive testing programs [Brinkman et al 1997]. (See also the National Society of Genetic Counselors resolution on genetic testing of children and the American Society of Human Genetics and American College of Medical Genetics points to consider : ethical, legal, and psychosocial implications of genetic testing in children and adolescents.)

CAG Repeat Size
Median Age at Onset (Years)

Range of Age
of Onset
(95% Confidence Interval) (Years)

39 repeats
66 yrs
(59-72) yrs
40 repeats
59 yrs
(56-61) yrs
41 repeats
54 yrs
(52-56) yrs
42 repeats
49 yrs
(48-50) yrs
43 repeats
44 yrs
(42-45) yrs
44 repeats
42 yrs
(40-43) yrs
45 repeats
37 yrs
(36-39) yrs
46 repeats
36 yrs
(35-37) yrs
47 repeats
33 yrs
(31-35) yrs
48 repeats
32 yrs
(30-34) yrs
49 repeats
28 yrs
(25-32) yrs
50 repeats
27 yrs
(24-30) yrs

From Brinkman et al 1997

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www.genetests.org
Children's Health System and University of Washington, Seattle

Posted by Dave at November 23, 2003 07:46 AM

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