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PubMed This is a summary of 23 peer-reviewed journal articles Updated
Genetics

Genetic Testing: A Roadmap for You and Your Family

At a Glance

Genetic testing for Familial Adenomatous Polyposis (FAP) involves diagnostic testing to find the APC gene mutation in the first affected family member, followed by predictive testing for relatives. Children of an affected parent are typically tested between ages 10 and 12 to begin screenings.

Confirming a diagnosis of Familial Adenomatous Polyposis (FAP) involves a combination of clinical findings—what a doctor sees during an exam—and genetic testing, which looks directly at your DNA [1][2]. This process not only confirms the condition for you but also creates a “genetic map” that can protect your family members [3][4].

Clinical Diagnosis

A doctor may first suspect FAP based on a clinical diagnosis. This typically happens when a colonoscopy reveals a high number of polyps [5][6]:

  • Classic FAP: Usually diagnosed when more than 100 adenomatous polyps are found in the colon [7][5].
  • Attenuated FAP (AFAP): Suspected when there are between 10 and 100 polyps, often appearing later in life [7][8].

The Two Types of Genetic Testing

Genetic testing for FAP primarily focuses on the APC gene. There are two main ways this testing is used [1][9]:

1. Diagnostic Testing

This is performed on the first person in a family suspected of having FAP (often called the index case or proband) [1][10]. The goal is to find the specific “pathogenic variant” (disease-causing mutation) that is responsible for the polyps [1][6]. If no APC mutation is found but the person has many polyps, doctors may also test the MUTYH gene [11][12].

2. Predictive Testing

Once a specific mutation is found in the index case, other family members can have predictive testing [9][3]. This test is highly accurate because the lab knows exactly which “typo” to look for.

  • If the test is positive: The person has inherited the gene and needs to follow the FAP surveillance roadmap [4][13].
  • If the test is negative: The person did not inherit the family’s FAP mutation and usually carries the same risk of colon cancer as the general population [3][4].

Testing Children

One of the most difficult parts of an FAP diagnosis is deciding when to test children. Because polyps can begin to form in the early teens, guidelines generally recommend genetic testing for children of an affected parent by age 10 to 12 [13][14].

In rare cases, testing may be done even earlier—sometimes in infancy—if there is a family history of hepatoblastoma (a rare childhood liver cancer associated with FAP), as these children may need liver screenings starting shortly after birth [15][16].

Understanding “Uncertain” Results

Sometimes a genetic test finds a change in the APC gene, but scientists don’t yet know if that change causes disease. This is called a Variant of Uncertain Significance (VUS) [17][18].

  • It is not a “yes”: A VUS should not be used to make major medical decisions, like surgery, because it might turn out to be a harmless, natural variation [19][20].
  • Management: If you have a VUS, doctors will usually base your care on your actual symptoms and your family history rather than the test result itself [20][21].
  • Updates: Over time, as more people are tested, researchers often “reclassify” these variants as either harmful or benign [17][22].

Genetic testing is a powerful tool, but it is best navigated with a genetic counselor. They can help you understand what the results mean for your health, your family, and your future [4][23].

Common questions in this guide

What is the difference between diagnostic and predictive genetic testing for FAP?
Diagnostic testing is done on the first person in a family suspected of having FAP to find the specific disease-causing mutation. Once that mutation is identified, predictive testing is used to see if other family members have inherited that exact same genetic change.
When should children be tested for FAP?
Guidelines generally recommend genetic testing for children of a parent with FAP between the ages of 10 and 12. In rare families with a history of hepatoblastoma, testing may be done in infancy to start liver screenings early.
What does a Variant of Uncertain Significance (VUS) mean on my APC gene test?
A VUS means a change was found in your APC gene, but scientists do not yet know if it causes FAP or if it is just a harmless natural variation. You should not make major medical decisions based solely on a VUS result, as it may be reclassified over time.
What if I have hundreds of colon polyps but my APC genetic test is negative?
If you have many polyps but no APC mutation is found, your doctor may recommend testing other genes, such as the MUTYH gene. Mutations in this gene can also cause a high number of polyps and require a specific surveillance plan.
Do I still have a high risk of colon cancer if my predictive FAP test is negative?
If your predictive test is negative, it means you did not inherit your family's specific FAP mutation. You generally carry the same risk for colon cancer as the general population and do not need to follow the intensive FAP surveillance roadmap.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Was a definitive 'pathogenic' mutation found in my APC gene, or was it a 'variant of uncertain significance' (VUS)?
  2. 2.Now that my mutation is identified, what is the specific process for testing my children or siblings (cascade testing)?
  3. 3.At what exact age do you recommend my children have their first genetic test?
  4. 4.If my child's genetic test is positive, when will they need to start their first colonoscopy or liver screening?
  5. 5.If my genetic test was negative but I have hundreds of polyps, should we test for MUTYH or other genes?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (23)
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This page provides educational information about genetic testing for Familial Adenomatous Polyposis (FAP). It does not replace professional medical advice, and you should always consult a genetic counselor or physician regarding your family's specific testing needs.

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