3.8 Predictive & Pre-symptomatic Testing
Key Takeaways
- Predictive/pre-symptomatic testing evaluates currently asymptomatic people for a known or suspected future Mendelian risk, often after a familial pathogenic variant is identified.
- Huntington disease–style protocols use multi-visit pretest counseling, informed consent, psychosocial assessment, and results disclosure with follow-up—not same-day drive-through testing.
- Professional guidance generally discourages predictive testing of asymptomatic minors for untreatable adult-onset conditions; deferral preserves the child’s future autonomy and right not to know.
- The right not to know is a valid outcome; declining testing after counseling is an acceptable informed decision.
- Actionable adult-onset syndromes (for example, some cancer predisposition genes) may justify different timing than non-actionable neurodegenerative predictions—but autonomy and psychosocial readiness still govern process.
Definitions that boards expect you to separate
| Term | Meaning | Example |
|---|---|---|
| Pre-symptomatic testing | Testing an asymptomatic person for a genotype that will (nearly) inevitably lead to disease if penetrance is complete | HTT CAG expansion testing in an adult child of a parent with Huntington disease |
| Predictive testing | Testing for a genotype that confers elevated future risk with incomplete penetrance | Pathogenic BRCA1 testing in an unaffected adult relative |
| Diagnostic testing | Testing someone already affected to explain disease | Panel testing in a woman with breast cancer |
| Susceptibility / PRS testing | Complex-trait risk stratification, usually modest effects | Polygenic risk scores (Domain 2B)—not classic Mendelian predictive protocols |
In practice, many clinicians say “predictive testing” for both complete and incomplete penetrance adult-onset scenarios. On exam items, focus on asymptomatic status + future disease risk + counseling process.
When predictive testing is appropriate
Typical prerequisites:
- A clearly defined familial risk (known familial P/LP variant preferred; sometimes a clinical diagnosis in the family with a testable gene).
- The consultand is asymptomatic (or not diagnosed with the condition of interest).
- Testing will be performed in a clinical laboratory with appropriate consent—not as an informal research add-on without disclosure planning.
- The person has capacity (or appropriate surrogate framework) and has engaged in pretest counseling.
- There is a plan for results disclosure, support, and medical follow-up if positive.
If no familial variant is known, testing an unaffected person with a “large panel” may be lower yield and harder to interpret—prefer testing an affected relative first when possible (diagnostic strategy from 3.6), then cascade.
Huntington-style multi-visit protocols (teaching gold standard)
Huntington disease (HD) predictive testing became the prototype for high-stakes adult-onset predictive programs. Core elements (conceptual—exact visit counts vary by clinic):
| Phase | Typical content |
|---|---|
| Pretest visit(s) | Motives for testing; knowledge of HD; inheritance; penetrance; limitations; insurance/psychosocial implications; support person; mental health screen |
| Informed consent | Voluntary nature; right to delay or decline; result possibilities (positive, negative, sometimes intermediate alleles); nondisclosure options |
| Sample collection | Often separated in time from initial counseling to reduce impulsive testing |
| Results visit | In-person (or thoughtfully planned synchronous) disclosure; avoid voicemail-only results for HD-like contexts |
| Follow-up | Short-interval psychosocial check-in regardless of result; referral pathways |
Why the process is slow on purpose: Predictive results can trigger grief, survivor guilt (negative result while siblings are positive), depression, suicidal ideation risk in vulnerable persons, relationship rupture, and irrevocable loss of the “right not to know.” The protocol is a safety and autonomy structure, not bureaucracy for its own sake.
Motives counseling — explore before drawing blood
Common motives: life planning, reproductive decisions, career/financial planning, relieving uncertainty, or pressure from relatives/partners. Counselors help distinguish intrinsic, informed motivation from coercion. A person who wants testing “only because my sibling did” may need more exploration.
Right not to know
The right not to know means a person may refuse predictive information even when relatives are testing or when a clinician thinks knowledge would be “useful.” Respecting refusal after balanced education is ethically standard. Related practices:
- Do not force results onto someone who withdraws consent before disclosure.
- Be careful with charting and family conferences so one relative’s result does not involuntarily disclose another’s status.
- In research or genomic sequencing contexts, discuss opt-out preferences for secondary findings (ties to Domain 3A/5).
Asymptomatic minors: the central debate
Professional statements from pediatrics and genetics organizations have long urged deferral of predictive testing in asymptomatic minors for adult-onset conditions that have no childhood interventions, especially untreatable neurodegenerative disease. Reasons:
| Argument for deferral | Plain-language meaning |
|---|---|
| Future autonomy | Adult-self should choose whether to know |
| Right not to know | Child cannot exercise refusal meaningfully |
| Psychosocial harm | Stigma, altered parenting, anxiety, insurance/privacy concerns |
| Low medical benefit in childhood | No change in pediatric management |
Important exceptions / nuances boards may test
- Actionable childhood management: If knowing genotype changes childhood surveillance or therapy (for example, certain cancer predisposition syndromes with childhood risks—familial adenomatous polyposis, Li-Fraumeni in appropriate contexts), testing may be offered on a medical-benefit timeline rather than deferred to adulthood.
- Diagnostic testing in a symptomatic child is not the same as predictive testing of a healthy child.
- Adolescent assent: When testing is medically indicated or carefully considered in teens, include age-appropriate assent and assess coercion.
- Carrier testing in minors for reproductive knowledge alone is generally deferred until adulthood/reproductive maturity unless there is a compelling reason (overlaps next section).
Exam pearl: The stem’s key is whether the minor is symptomatic, whether the condition has childhood actionability, and whether the request is purely for adult-onset nonactionable prediction.
Predictive testing in actionable hereditary cancer syndromes
Unaffected adults in families with pathogenic BRCA1/2, Lynch, TP53, etc., often pursue predictive/cascade testing because results change screening, risk-reducing surgery discussions, and family planning. Process still includes pretest counseling, but clinics may not mirror every HD visit element. Nonetheless:
- Confirm the familial variant when known.
- Discuss GINA and insurance limitations (Domain 5) without over-reassuring beyond the law’s scope.
- Plan medical management referrals before or immediately with positive results.
- Negative familial-variant results usually return the person to near-population risk for that gene—family history from other causes may still matter.
Psychosocial outcomes to anticipate
| Result | Possible reactions |
|---|---|
| Positive | Anticipatory grief, hypervigilance, empowerment via action plans, family communication burden |
| Negative | Relief, survivor guilt, unexpected identity shift (“I thought I would be positive”), minimization of residual family risks |
| Uninformative / VUS (if broad testing used) | Frustration, anxiety; avoid predictive medical decisions based on VUS |
Worked scenarios
Scenario A — HD protocol
A 28-year-old whose parent has genetically confirmed HD requests testing “today, just draw the labs.” Best response: engage in structured pretest counseling, assess psychosocial readiness and support, explain multi-step process and right to defer—do not treat it as a routine same-visit consumer test.
Scenario B — minor, nonactionable adult-onset
Parents want HD testing for their healthy 8-year-old “so we can plan.” Best counseling direction: explain professional recommendations to defer until adulthood (or until the person can consent), focusing on the child’s future autonomy and lack of childhood medical benefit.
Scenario C — minor, actionable
A family has a pathogenic APC variant (FAP). An asymptomatic child is approaching the age when colonoscopy surveillance would begin if positive. Testing may be appropriate to guide childhood management, with pediatric-focused counseling and assent as age-appropriate.
Exam traps
- Equating all predictive testing with refusal to test minors (actionability matters).
- Offering predictive results by unsecured email without a disclosure plan in high-stakes contexts.
- Testing unaffected people with large panels before identifying a familial variant when an affected relative is available.
- Treating a person’s decline of testing as noncompliance rather than a valid exercise of the right not to know.
Practice checkpoints
- Define predictive/pre-symptomatic vs diagnostic indications.
- List core Huntington-protocol elements and why they exist.
- Apply the asymptomatic-minor deferral rule and name actionability exceptions.
- Counsel the right not to know without abandoning education.
Parents request predictive Huntington disease testing for their healthy 7-year-old because a grandparent is affected. What is the most appropriate counseling emphasis?
Which element is most characteristic of a Huntington-style predictive testing protocol?
An asymptomatic adult relative in a family with a known pathogenic BRCA2 variant declines testing after thorough counseling. What is the best interpretation?
In which situation is predictive genetic testing of an asymptomatic child most likely to be considered medically appropriate rather than deferred?