Certain pMMR colorectal cancer patients should undergo additional MSI-PCR testing to reduce the risk of misdiagnosing MSI-H and Lynch syndrome

This study analyzed colorectal cancers with proficient mismatch repair by immunohistochemistry but high microsatellite instability by PCR to characterize when discordance matters clinically. The authors identified a subset of pMMR patients who still warrant additional MSI-PCR testing and proposed a targeted strategy to reduce missed MSI-H tumors and Lynch syndrome diagnoses.