Association of Primary Resistance to Immune Checkpoint Inhibitors in Metastatic Colorectal Cancer With Misdiagnosis of Microsatellite Instability or Mismatch Repair Deficiency Status

Primary resistance of mCRC displaying MSI or dMMR to immune checkpoint inhibitors is due mainly to misdiagnosis of their MSI or dMMR status. Larger studies are required to confirm these findings. Microsatellite instability or dMMR status should be tested routinely using both immunohistochemistry and polymerase chain reaction methods prior to treatment with immune checkpoint inhibitors.