How is the Presymptomatic Test Conducted?

How is the Presymptomatic Test Conducted?

Information provided by the
National Institute of Neurological Disorders and Stroke
National Institutes of Health

An individual who wishes to be tested should contact the nearest testing center. (A list of such centers can be obtained from the Huntington Disease Society of America at 1-800-345-HDSA.)
The testing process should include several components. Most testing programs include a neurological examination, pretest counseling, and followup. The purpose of the neurological examination is to determine whether or not the person requesting testing is showing any clinical symptoms of HD.
It is important to remember that if an individual is showing even slight symptoms of HD, he or she risks being diagnosed with the disease during the neurological examination, even before the genetic test.
During pretest counseling, the individual will learn about HD, about his or her own level of risk, and about the testing procedure. The person will be told about the test's limitations, the accuracy of the test, and possible outcomes. He or she can then weigh the risks and benefits of testing and may even decide at that time against pursuing further testing.

If a person decides to be tested, a team of highly trained specialists will be involved, which may include neurologists, genetic counselors, social workers, psychiatrists, and psychologists. This team of professionals helps the at-risk person decide if testing is the right thing to do and carefully prepares the person for a negative, positive, or inconclusive test result.

Individuals who decide to continue the testing process should be accompanied to counseling sessions by a spouse, a friend, or a relative who is not at risk. Other interested family members may participate in the counseling sessions if the individual being tested so desires.

The genetic testing itself involves donating a small sample of blood that is screened in the laboratory for the presence or absence of the HD mutation. Testing may require a sample of DNA from a closely related affected relative, preferably a parent, for the purpose of confirming the diagnosis of HD in the family. This is especially important if the family history for HD is unclear or unusual in some way.

Results of the test should be given only in person and only to the individual being tested. Test results are confidential. Regardless of test results, followup is recommended.

In order to protect the interests of minors, including confidentiality, testing is not recommended for those under the age of 18 unless there is a compelling medical reason (for example, the child is exhibiting symptoms).

Testing of a fetus (prenatal testing) presents special challenges and risks; in fact some centers do not perform genetic testing on fetuses. Because a positive test result using direct genetic testing means the at-risk parent is also a gene carrier, at-risk individuals who are considering a pregnancy are advised to seek genetic counseling prior to conception.

Some at-risk parents may wish to know the risk to their fetus but not their own. In this situation, parents may opt for prenatal testing using linked DNA markers rather than direct gene testing. In this case, testing does not look for the HD gene itself but instead indicates whether or not the fetus has inherited a chromosome 4 from the affected grandparent or from the unaffected grandparent on the side of the family with HD.
If the test shows that the fetus has inherited a chromosome 4 from the affected grandparent, the parents then learn that the fetus's risk is the same as the parent (50-50), but they learn nothing new about the parent's risk.
If the test shows that the fetus has inherited a chromosome 4 from the unaffected grandparent, the risk to the fetus is very low (less than 1%) in most cases.

Another option open to parents is in vitro fertilization with preimplantation screening. In this procedure, embryos are screened to determine which ones carry the HD mutation. Embryos determined not to have the HD gene mutation are then implanted in the woman's uterus.

In terms of emotional and practical consequences, not only for the individual taking the test but for his or her entire family, testing is enormously complex and has been surrounded by considerable controversy. For example, people with a positive test result may risk losing health and life insurance, suffer loss of employment, and other liabilities.
People undergoing testing may wish to cover the cost themselves, since coverage by an insurer may lead to loss of health insurance in the event of a positive result, although this may change in the future.

With the participation of health professionals and people from families with HD, scientists have developed testing guidelines. All individuals seeking a genetic test should obtain a copy of these guidelines, either from their testing center or from the organizations listed on the card in the back of this brochure. These organizations have information on sites that perform testing using the established procedures and they strongly recommend that individuals avoid testing that does not adhere to these guidelines.