HOW IS GENETIC TESTING USED?

Clinically applicable genetic tests may be used for:

  • Diagnostic testing
  • Predictive testing
  • Carrier testing
  • Prenatal testing
  • Preimplantation testing
  • Newborn screening

Research tests generally do not give clinically applicable results.

Diagnostic Testing

Diagnostic testing is used to confirm or rule out a known or suspected genetic disorder in a symptomatic individual.

Points to consider:

  • DNA testing may yield diagnostic information at a lower cost and with less risk than other procedures.
  • Diagnostic testing is appropriate in symptomatic individuals of any age.
  • Confirming a diagnosis may alter medical management for the individual.
  • Diagnostic testing of an individual may have reproductive or psychosocial implications for other family members as well.
  • Establishing a diagnosis may require more than one type of genetic test.
  • DNA testing may not always be the best way to establish a clinical diagnosis.

Predictive Testing

Predictive testing is offered to asymptomatic individuals with a family history of a genetic disorder. Predictive testing is of two types: presymptomatic (eventual development of symptoms is certain when the gene mutation is present, e.g., Huntington disease) and predispositional (eventual development of symptoms is likely but not certain when the gene mutation is present, e.g., breast cancer).

Points to consider:

  • Predictive testing is MEDICALLY INDICATED if early diagnosis allows interventions which reduce morbidity or mortality.
  • Even in the absence of medical indications, predictive testing can influence life planning decisions.
  • Because predictive testing can have psychological ramifications, careful patient assessment, counseling, and follow-up are important.
  • Many laboratories will not proceed with predictive testing without proof of informed consent and genetic counseling.
  • Identification of the specific gene mutation in an affected relative or establishment of linkage within the family should precede predictive testing.
  • Predictive testing of asymptomatic children at risk for adult onset disorders is strongly discouraged when no medical intervention is available.

Carrier Testing

Carrier testing is performed to identify individuals who have a gene mutation for a disorder inherited in an autosomal recessive or X-linked recessive manner. Carriers usually do not themselves have symptoms related to the gene mutation. Carrier testing is offered to individuals who have family members with a genetic condition, family members of an identified carrier, and individuals in ethnic or racial groups known to have a higher carrier rate for a particular condition.

Points to consider:

  • Identifying carriers allows reproductive choices.
  • Genetic counseling and education should accompany carrier testing because of the potential for personal and social concerns.
  • Molecular genetic testing of an affected family member may be required to determine the disease-causing mutation(s) present in the family.
  • In some situations, DNA testing may not be the primary way of determining carrier status.
  • Carrier testing can improve risk assessment for members of racial and ethnic groups more likely to be carriers for certain genetic conditions.

Prenatal Testing

Prenatal testing is performed during a pregnancy to assess the health status of a fetus. Prenatal diagnostic tests are offered when there is an increased risk of having a child with a genetic condition due to maternal age, family history, ethnicity, or suggestive multiple marker screen or fetal ultrasound examination. Routine prenatal diagnostic test procedures are amniocentesis and chorionic villus sampling (CVS). More specialized procedures include placental biopsy, periumbilical blood sampling (PUBS), and fetoscopy with fetal skin biopsy.

Points to consider:

  • A laboratory that performs the disease-specific test of interest must be identified before any prenatal diagnostic test procedure is offered.
  • All prenatal diagnostic test procedures have an associated risk to the fetus and the pregnancy; therefore, informed consent is required, most often in conjunction with genetic counseling.
  • In most cases, before prenatal diagnosis using molecular genetic testing can be offered, specific gene mutation(s) must be identified in an affected relative or carrier parent(s).
  • Prenatal testing for adult-onset conditions is controversial. Individuals seeking prenatal diagnosis for these conditions should be referred to a professional trained in genetic counseling for a complete discussion of the issues.

Preimplantation Testing (Preimplantation Genetic Diagnosis)

Preimplantation testing is performed on early embryos resulting from in vitro fertilization in order to decrease the chance of a particular genetic condition occurring in the fetus. It is generally offered to couples with a high chance of having a child with a serious disorder. Preimplantation testing provides an alternative to prenatal diagnosis and termination of affected pregnancies.

Points to consider:

  • Preimplantation testing is only performed at a few centers and is only available for a limited number of disorders.
  • Preimplantation testing is not possible in some cases due to difficulty in obtaining eggs or early embryos and problems with DNA analysis procedures.
  • Due to possible errors in preimplantation diagnosis, traditional prenatal diagnostic methods are recommended to monitor these pregnancies.
  • The cost of preimplantation testing is very high and is usually not covered by insurance.

Newborn Screening

Newborn screening identifies individuals who have an increased chance of having a specific genetic disorder so that treatment can be started as soon as possible.

Points to consider:

  • Newborn screening programs are usually legally mandated and vary from state to state.
  • Newborn screening is performed routinely at birth, unless specifically refused by the parents in writing.
  • Screening tests are not designed to be diagnostic, but to identify individuals who may be candidates for further diagnostic tests.
  • Many parents do not realize that newborn screening has been done (or which tests were included), even if they signed a consent form when their child was born.
  • Education is necessary with positive screening results in order to avoid misunderstandings, anxiety, and discrimination.

Modified from: GeneTests: Medical Genetics Information Resource (database online). Educational Materials: About Genetic Services. Copyright, University of Washington, Seattle. 1993-2006. Available at http://www.genetests.org. Accessed January 5, 2006.