Diagnosis combines clinical, endoscopic, and genetic elements, each serving a distinct purpose:

*Colonoscopy remains the diagnostic cornerstone, both for establishing the diagnosis in a symptomatic or at-risk individual and for the ongoing surveillance that follows. The finding of more than one hundred adenomatous polyps is considered diagnostic of classic disease in its own right, even before genetic confirmation.
*Genetic testing for pathogenic variants in the APC gene confirms the molecular diagnosis, clarifies whether the presentation is classic or attenuated based on variant location, and, critically, enables predictive testing of at-risk relatives once a family’s specific variant is known. Predictive testing is generally offered from around age ten to twelve for families with classic disease, allowing surveillance to begin in only those children who actually carry the variant.
*Genetic counseling, both before testing (pre-test counseling) and after results are available (post-test counseling), is considered an integral part of diagnosis rather than an optional add-on, given the autosomal dominant inheritance pattern, the direct implications for siblings and children, the psychological burden of a near-certain cancer diagnosis in the absence of intervention, and the complexity of explaining reduced but still elevated risk in attenuated or variant-negative familial cases.

Management is lifelong and multidisciplinary, structured around three broad phases:

Surveillance phase:

*Colonoscopy is initiated around age ten to twelve in known or at-risk individuals with classic disease, or somewhat later, around the late teens to early twenties, in attenuated disease, with frequency (commonly annual once polyps are found) determined by polyp number, size, and histology.
*Upper gastrointestinal endoscopy to assess the duodenum and stomach typically begins once colorectal polyps are established, or by the mid-twenties, with subsequent frequency guided by the Spigelman staging system, which grades duodenal polyp burden based on number, size, histological type, and degree of cellular change.
*Periodic thyroid palpation or ultrasound, and, in some protocols, annual physical examination for abdominal or soft-tissue masses to detect desmoid tumors early.

Surgical phase:

*Prophylactic colectomy is generally recommended once polyp burden becomes unmanageable through colonoscopic polypectomy alone, most commonly in the late teens to twenties for classic disease, though timing is individualized based on polyp number, size, and degree of cellular change on biopsy rather than age alone.
*Two main surgical approaches are used: restorative proctocolectomy with formation of an ileal pouch and anastomosis to the anal canal (removing the entire colon and rectum while preserving bowel continuity), or total colectomy with ileorectal anastomosis (removing the colon but preserving the rectum, requiring continued rectal surveillance thereafter). The choice depends on rectal polyp burden, patient preference, and anticipated ability to comply with lifelong rectal surveillance if the rectum is retained.

Lifelong follow-up phase:

*Surveillance continues after surgery regardless of approach, since retained rectal tissue, the ileal pouch, the duodenum, and the thyroid all remain at ongoing risk.
*A multidisciplinary team, typically comprising a clinical geneticist, gastroenterologist, colorectal surgeon, and genetic counselor, and often an endocrinologist and dietitian, coordinates the individualized timing of surgery and the surveillance schedule to the person’s specific APC gene variant, polyp burden, and life stage, adjusting the plan as new findings emerge over decades of follow-up.