The new Droplet Digital PCR Microsatellite Instability (MSI) RUO assay can be completed in one day on a ddPCR-based platform to quantify the level of MSI present in colorectal tumors. Using either a blood or FFPE sample, the assay quantitatively identifies mutations in five loci that lead to the identification of MSI status. Colorectal tumors that test positive for MSI-high are candidates for treatment with immune-checkpoint inhibitors. This new assay is simple, highly sensitive, and provides a standardized output that does not require a pathologist’s interpretation. The test can use patient blood samples in the absence of tumor tissue and does not require matched normal tissue or normal blood samples.