Major points and concepts to consider when offering pan-ethnic expanded carrier testing

In the September issue of Contemporary OB/GYN, Dr. Ronald Wapner authored 'A case for improved carrier screening,' which featured the following summarization of considerations relating to expanded carrier screening (ECS).

Dr. Wapner is the vice chair of research in the department of obstetrics and gynecology for Columbia University Irving Medical Center in New York and its director of reproductive genetics. His areas of expertise are fetal testing, fetal anomalies, genetic testing, and preterm birth prevention.

Major points and basic concepts to be considered in offering pan-ethnic expanded carrier testing^{1, 2}

• All individuals, regardless of race or ethnicity, should be offered screening for the same set of conditions.
• Carrier screening of any nature is voluntary, and it is reasonable to accept or decline.
• Results of genetic testing are confidential and protected in health insurance and employment by the Genetic Information Non-Discrimination Act of 2008.³
• The majority of conditions on current expanded panels are autosomal-recessive. However, some may be X-linked or autosomal-dominant single-gene conditions.
• Expanded carrier screening panels can include more than 400 genetic conditions; many of which are rare. Conditions included on expanded carrier screening panels vary in severity. Many are associated with significant adverse outcomes such as cognitive impairment, decreased life expectancy, and the need for medical or surgical intervention.
• Before testing, it is not practical or necessary to fully explain all of the clinical and test characteristics of each condition. Pretest education and consent should broadly describe the types of conditions being screened for and their common features as well as the limitations of screening. Educating patients before testing may be done verbally or by using other informational approaches such as pamphlets, videos, or online resources.

General concepts to be included in pretest counseling should include:

• Some conditions screened have less well-defined phenotypes.
• Because many conditions being screened are rare, disease prevalence, mutation frequencies, and detection rates may be imprecise, and residual risk estimations may not be reliable.
• Screen-negative results reduce the likelihood of the carrier state for the conditions, but a residual risk of being a carrier always remains i.e. A negative screen does not eliminate risk to offspring.

• Screening panels may change over time, and there may be differences in the conditions screened between laboratories. Despite this, rescreening with each pregnancy should not be offered or recommended.
• Pregnancy risk assessment depends on accurate knowledge of paternity. If the biological father is not available for carrier screening, accurate risk assessment for recessive conditions is not possible.
• Because expanded carrier screening includes a large number of disorders, it is common to identify carriers for one or more conditions. In most cases, being a carrier of an autosomal recessive condition has no clinical consequences for the individual carrier. If each partner is identified as a carrier of a different autosomal recessive condition, offspring are not likely to be affected.
• In some instances, an individual may learn that they have two pathogenic variants for a condition (homozygous or compound heterozygous) and thus learn through carrier screening that they have an autosomal-recessive condition that could affect their personal health.

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References

1. Edwards JG, Feldman G, Goldberg J, et al. Expanded carrier screening in reproductive medicine-points to consider: a joint statement of the American College of Medical Genetics and Genomics, American College of Obstetricians and Gynecologists, National Society of Genetic Counselors, Perinatal Quality Foundation, and Society for Maternal-Fetal Medicine. Obstet Gynecol. 2015;125(3):653-662.
2. Benn P, Chapman AR, Erickson K, et al. Obstetricians and gynecologists' practice and opinions of expanded carrier testing and noninvasive prenatal testing. Prenat Diagn. 2014;34(2):145-152.
3. Hogan GJ, Vysotskaia VS, Beauchamp KA, et al. Validation of an Expanded Carrier Screen that Optimizes Sensitivity via Full-Exon Sequencing and Panel-wide Copy Number Variant Identification. Clin Chem. 2018;64(7):1063-1073.