Chicago Center for Jewish Genetics Disorders - About Jewish Genetic Disorders

Predictive Testing

What is it: Some genetic disorders are called late-onset disorders and they do not appear at birth or even in childhood. Instead they become apparent when an individual is an adult. If a specific mutated gene is identified that is responsible for a late-onset disease, a person from an affected family can be tested before symptoms appear. This is called predictive testing and it can be done for such disorders as breast, ovarian, and colon cancer, Huntington disease, and heart disease. This category includes two types of tests, predictive and presymptomatic:

Predictive Testing is for disorders in which eventual development of symptoms is certain when the gene mutation is present. For example, a disease-causing mutation in the Huntingtin gene is assumed to guarantee the eventual development of Huntington disease. This kind of testing provides a yes-or-no answer for the eventual manifestation of the disease.
Presymptomatic Testing (also called predispositional testing) is for disorders in which eventual development of symptoms is likely but not certain when the gene mutation is present. Predictive test results do not necessarily mean that the disease will inevitably occur (even with a positive result) or remain absent (even with a negative result). This type of testing does not give a yes-or-no answer for developing a disorder but rather a modification of the initial risk. An example of this is a mutation in the BRCA1 or BRCA2 gene and the development breast cancer. Seven percent of all women in the United States will develop breast cancer by the age of 70. In contrast, a woman who tests positive for a BRCA1 mutation has a 55-85% chance of developing breast cancer by the age of 70. However, this also means that a woman with a BRCA1 mutation would have a 15-45% chance of NOT developing breast cancer by age 70 despite having a mutation.

When is it done: Predictive testing is offered to asymptomatic individuals with a family history of a genetic disorder. Predictive testing can ethically be done in children only if early diagnosis, before adulthood, allows for interventions that reduce morbidity or mortality. An example of this is Familial Adenomatous Polyposis, classical type. However, predictive testing of asymptomatic children at risk for adult onset disorders is strongly discouraged when no medical intervention is available (ACMG Policy Statement, http://www.faseb.org/genetics/acmg/pol-13.htm). This is discussed further in The Ethics of Mendelian Disorders and Multifactorial (Predispositional) Disorders.

Benefits of testing: An individual may wish to make medical decisions based upon the outcome of the test, for both positive and negative results.

Other things to consider:

Identification of the specific gene mutation in an affected relative or establishment of linkage within the family should precede predictive testing. It is also important to remember that genetic information has consequences for the entire family, not just the individual being tested. Some individuals may not wish to know that they are at increased risk for a disorder.
In addition to medical care decisions, predictive testing has psychological ramifications and can affect an individual's lifestyle and reproductive choices. There are also concerns about insurance or employment discrimination in cases where a test comes back positive. For a discussion of these issues, please see Non-technical Aspects of Genetic Testing.

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