We recommend APC mosaicism testing at least in patients negative for germline pathogenic variants with (1) ≥20 adenomas before the age of 60 or (2) ≥30 adenomas before the age of 70. Regular colonoscopy and at least 1 gastroduodenoscopy should be offered to APC mosaic patients, with frequency of follow-up based on findings. Offering germline testing for offspring should be considered.
Terlouw et al. (Sun,) studied this question.