What are the key clinical features that suggest a diagnosis of Familial Adenomatous Polyposis (FAP) in a patient?

Key clinical features suggesting a diagnosis of Familial Adenomatous Polyposis (FAP) include the presence of hundreds to thousands of colorectal adenomatous polyps, typically developing in adolescence or early adulthood. These polyps are usually numerous and distributed throughout the colon and rectum, often leading to colorectal cancer if untreated ⁴. Patients may present with symptoms such as rectal bleeding, diarrhoea, abdominal pain, or anaemia due to chronic blood loss. Additionally, a strong family history of colorectal cancer or polyposis is a critical clinical clue ⁴.

Extra-colonic manifestations are also important clinical features that support the diagnosis of FAP. These include congenital hypertrophy of the retinal pigment epithelium (CHRPE), desmoid tumours, osteomas, dental abnormalities, and upper gastrointestinal polyps, particularly in the duodenum and stomach (Joo et al., 2025). The presence of these extracolonic signs alongside colonic polyposis strengthens the suspicion of FAP.

Genetic testing for pathogenic variants in the APC gene is confirmatory but clinical diagnosis relies heavily on the characteristic phenotype of numerous adenomatous polyps and family history. Early identification is crucial due to the near 100% risk of colorectal cancer without prophylactic colectomy ⁴; (Lal and Gallinger, 2000).