Is there a genetic blood test for Familial Adenomatous Polyposis (FAP)?

Genetic Testing for Familial Adenomatous Polyposis (FAP)

Yes, genetic blood testing is available for Familial Adenomatous Polyposis (FAP) through DNA sequencing and large rearrangement analysis of the APC gene and MUTYH gene. 1

Testing Approach for FAP

Who Should Be Tested

• Individuals with >100 colorectal adenomas (classical FAP phenotype) 1
• Individuals with 10-99 adenomas (possible attenuated FAP) 1
• First-degree relatives of individuals with known FAP mutations 1, 2
• Children or young adults with family history of FAP (testing can begin at age 10) 1

Genetic Testing Process

1. Initial testing should be performed on an affected individual to identify the specific mutation 1
2. Blood sample collection for DNA extraction from peripheral white blood cells 1
3. Testing methods include:
   • Full APC gene sequencing
   • Large rearrangement analysis
   • If APC testing is negative, MUTYH gene testing should be considered 1

Testing Yield

The likelihood of identifying an APC mutation varies by adenoma count 1:

• 80% in individuals with >1000 adenomas
• 56% in individuals with 100-999 adenomas
• 10% in individuals with 20-99 adenomas
• 5% in individuals with 10-19 adenomas

Clinical Implications of Genetic Testing

Benefits of Testing

• Allows for targeted surveillance of mutation carriers only 1
• Enables early intervention to prevent colorectal cancer (nearly 100% lifetime risk) 1
• Identifies family members who do not carry the mutation and can follow average-risk screening 1
• Guides appropriate timing of prophylactic colectomy 1, 2
• Informs surveillance for extracolonic manifestations 2

Management Based on Test Results

• APC mutation positive: Annual sigmoidoscopy/colonoscopy beginning at age 10-15 years 1, 2
• APC mutation negative (in a family with known mutation): Average-risk screening 1
• No mutation identified in family: Age-based surveillance protocol still needed 1

Important Considerations

Genetic Counseling

Genetic counseling is essential before and after testing to 3, 4:

• Explain the implications of positive and negative results
• Address psychological impacts of testing
• Discuss potential insurance discrimination concerns
• Help with family communication about genetic risk

Testing in Children

• Testing can begin at age 10 years 1
• Special counseling considerations are needed for minors 4
• Benefits include early identification for targeted surveillance

Common Pitfalls to Avoid

• Not testing an affected family member first - this makes interpretation of negative results difficult 1
• Using sigmoidoscopy alone for attenuated FAP - full colonoscopy is required due to proximal polyp distribution 2
• One-size-fits-all screening approach - protocols should be tailored based on specific mutation and family history 2
• Inadequate post-testing surveillance - continued monitoring is essential even after genetic diagnosis 2

Genetic testing for FAP is a powerful tool for early identification of at-risk individuals, allowing for appropriate surveillance and timely intervention to prevent the nearly inevitable development of colorectal cancer in untreated patients.