Laboratory tests - genetic testing (appropriate use of)

Role

Genetic testing in gastroenterology is used selectively for specific, well-defined indications - hereditary cancer syndrome risk (particularly Lynch syndrome and familial adenomatous polyposis), coeliac disease risk exclusion (HLA-DQ2/DQ8), and confirming specific inherited metabolic/liver conditions
Appropriate use requires matching the test to a specific clinical question with a validated genetic basis, rather than broad, undirected testing - genetic testing in GI practice is targeted, not a screening tool applied indiscriminately

Lynch syndrome (hereditary non-polyposis colorectal cancer) - caused by mismatch repair gene mutations (MLH1, MSH2, MSH6, PMS2); tumour testing (microsatellite instability/immunohistochemistry for mismatch repair protein loss) is typically performed first as a screen, with germline genetic testing reserved for those with a positive tumour screen or strong clinical suspicion
Familial adenomatous polyposis (FAP) - APC gene mutation, causing hundreds to thousands of colorectal polyps and near-certain colorectal cancer without intervention if untreated
HLA-DQ2/DQ8 for coeliac disease - as above, useful mainly for its very high negative predictive value to exclude the diagnosis, not to confirm it

Genetic testing should follow appropriate genetic counselling (see obtaining written consent for genetic testing) given the implications for the patient and their relatives, and should be targeted to a validated indication rather than performed speculatively
Tumour-based screening (MSI/immunohistochemistry) before germline testing is the standard pathway for Lynch syndrome - it is more efficient and cost-effective than germline testing every patient with colorectal cancer up front

A confirmed Lynch syndrome or FAP diagnosis triggers intensive colonoscopic surveillance (starting at a younger age and more frequently than average-risk screening) and cascade testing of at-risk relatives, and for FAP specifically, consideration of prophylactic colectomy given the near-certain cancer risk without intervention

Lynch syndrome, familial adenomatous polyposis, coeliac disease (HLA exclusion testing), hereditary pancreatitis genetic testing

Genetic testing is most valuable when it changes management (surveillance intensity, prophylactic intervention, cascade family testing) - testing without a clear subsequent action plan provides information without clinical benefit and should be preceded by counselling about what a positive or negative result will actually mean for the patient's care

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