3.9 Carrier, Somatic & Research Testing

Carrier testing and carrier screening

Carrier testing usually means testing an individual for carrier status—often because a partner is a known carrier, a family history exists, or an ethnicity/ancestry risk is recognized. Carrier screening is the broader offer of testing to people without a known familial variant, typically preconception or prenatal, to identify reproductive risks for autosomal recessive and selected X-linked conditions.

Residual risk — never promise zero

Even after a negative screen, residual carrier risk persists because panels miss some pathogenic variants, rare genes are absent, and coverage is incomplete. Counseling formula conceptually:

Posterior carrier risk depends on prior (ancestry/family history) × test sensitivity for that gene.

If partner A is a known carrier and partner B screens negative for that gene with high detection rate, risk of an affected child falls substantially but is not absolute zero unless the test essentially excludes carrier status for practical purposes—still speak in residual-risk language.

Ethnicity-based history vs universal / expanded screening

Historical model

For decades, guidelines emphasized ethnicity-based offering (for example, Ashkenazi Jewish panels for Tay–Sachs, Canavan, familial dysautonomia; hemoglobinopathy screening by ancestry; cystic fibrosis with varying detection by ancestry). This model aimed resources at higher carrier frequencies but had problems:

• Self-reported ancestry is incomplete or mixed
• Equitable offer was inconsistent
• Many couples outside “targeted” groups still had risk
• Stigmatization and checklist medicine risks

Contemporary direction

Professional bodies (including ACMG practice resources on prenatal/preconception carrier screening) have moved toward broader, more universal offering of expanded panels (tiered gene lists in ACMG frameworks), rather than offering only if a patient “looks” like a high-risk ethnicity. Ancestry remains relevant for:

• Interpreting detection rates and residual risk
• Adding condition-specific tests historically enriched in a group when not on a given panel
• Counseling hemoglobinopathy risks and other population genetics nuances

Expanded carrier screening (ECS) counseling must cover: conditions vary by lab; some conditions are variable in severity; not all childhood-lethal; partner testing strategy if one is positive; and that ECS is not a substitute for newborn screening or for diagnostic testing of a symptomatic person.

Cascade after a carrier is identified

1. Educate about the condition and inheritance.
2. Offer partner testing—preferably sequencing/analysis appropriate to the gene, not an overly limited variant panel that misses partner risk.
3. If both carry pathogenic variants in the same gene, discuss reproductive options (PGT, prenatal diagnosis, donor gametes, acceptance of risk)—Domain 3C detail.
4. Offer testing to relatives who may share carrier status when appropriate.

Somatic (tumor) testing vs germline testing

Oncology increasingly orders tumor (somatic) genomic profiling to identify therapeutic targets (for example, EGFR in lung cancer, NTRK fusions, homologous recombination repair defects relevant to PARP inhibitors). Genetic counselors must prevent category errors:

Counseling rules of thumb

• A tumor report listing a BRCA1 variant is not automatically a hereditary diagnosis—confirm with germline testing when hereditary risk is possible or when guidelines recommend reflex germline evaluation.
• “Germline-focused” hereditary cancer panels and tumor-only assays answer different questions; do not tell relatives to change medical care based solely on an unconfirmed tumor call.
• Paired tumor–normal sequencing can help sort somatic vs germline in one workflow when available.
• CHIP (clonal hematopoiesis) and mosaicism can complicate blood-based findings—another reason to interpret in oncology-genetics context.

Worked scenario

Tumor sequencing in a 42-year-old with breast cancer shows a TP53 variant. Next best genetics step: evaluate for possible Li-Fraumeni germline risk with appropriate pretest counseling and germline confirmatory testing—not cascade testing of children based on the tumor report alone.

Research testing vs clinical testing

Hard counseling line: If a patient needs a result for medical or reproductive decisions, pursue clinical testing. Research participation can be complementary (gene discovery after negative clinical exome) but should not be sold as equivalent care. When research returns a candidate finding, confirm in a clinical laboratory before medical management or cascade testing.

Consent themes unique to research genomics

• Possibility of no personal results
• Data sharing (controlled-access databases), re-identification risks, and future recontact
• Secondary findings policies differing from ACMG clinical secondary findings menus
• Right to withdraw (data already shared may not be fully retractable)
• Whether findings will be reanalyzed over time

Putting Domain 3B test types together

Board stems often mix two categories (tumor finding → germline counseling; ECS positive → partner testing; research candidate variant → clinical confirmation). Name the category first, then choose the next step.

Exam traps

• Telling an AR carrier they “have the disease.”
• Claiming negative ECS means zero risk of an affected child.
• Cascading relatives from tumor-only variants without germline confirmation.
• Using a research study as the only pathway for urgent clinical management decisions without explaining limitations.
• Offering ethnicity-only screening and refusing ECS discussion solely based on appearance.

Practice checkpoints

1. Explain residual risk after negative carrier screening.
2. Contrast ethnicity-targeted vs universal expanded offering.
3. Separate somatic tumor results from germline cascade implications.
4. State when research findings require clinical confirmation.