Up to 22% of all gastric adenocarcinomas are of the microsatellite instability (MSI) subtype. This subtype may not benefit from conventional adjuvant chemotherapy but does respond to novel immunotherapies. A proportion of MSI gastric adenocarcinomas are associated with inherited disease, inferring an opportunity to screen for further cancers in the patient while also identifying at-risk relatives. Although the importance of MSI subtyping is clear, the methods of detection vary. The main techniques are MSI testing and mismatch repair (MMR) immunohistochemistry (IHC). This study compares a novel Idylla (Biocartis) MSI assay to MMR IHC across 50 biopsies of gastric adenocarcinoma. The methods were concordant across 48 cases. The two discrepant results demonstrated known difficulties in the interpretation of IHC. Idylla MSI testing presents several advantages over MMR IHC but both methods are well established in detecting this subtype of gastric adenocarcinoma. The methods are best regarded as complementary tests, performing most optimally when combined.

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