Results from an institutional universal reflex testing program for MSI/MMR in colorectal cancer.

796Background: In colorectal cancer (CRC), clinical guidelines and immunotherapy treatment selection requires knowledge of tumor DNA mismatch repair gene deficiencies (dMMR) or accumulation of microsatellite repeats through genomic errors (MSI-H). Tumors harboring dMMR/MSI-H are found in 15- 20% of early stage CRC while the prevalence is ~5% in metastatic disease. Reflex testing of CRC to identify these important subsets has been proposed as a system-solution to improve identification. We present a large, single-institutional database of CRC reflexively profiled for MSI/MMR status at the University of Florida (UF). Methods: Beginning in 2009, stage IV CRC underwent reflex testing for MSI/MMR status. Earlier stage CRC testing began in subsequent years. For all new CRC diagnosed at UF, concurrent testing for dMMR by IHC (MLH1, PMS2, MSH2, MSH6), MSI by PCR (Promega MSI kit) and NGS is performed with appropriate positive and negative controls. IHC protein loss is confirmed by second GI pathologist. MSI is no...