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Microsatellite instability (MSI) testing is used to screen for Lynch syndrome. The current technique for MSI determination requires DNA from normal and neoplastic tissue and is expensive and laborious. Five quasi-monomorphic markers (NR-21, BAT-25, MONO-27, NR-24, and BAT-26) are included in the Promega MSI analysis kit. With the working hypothesis that this 5-marker panel can accurately determine the MSI status of colorectal tumors without using paired control DNA, we evaluated 478 colorectal tumors and divided them into a test group (N=172, colorectal adenocarcinomas) and a validation group (N=306 including 179 colorectal adenocarcinomas and 127 adenomas). The quasi-monomorphic variation range of each marker was generated from the test group (172 normal samples) and used as a reference value in the subsequent interpretation of MSI status in the test and validation groups. Considering the MSI result using a 5-marker panel with paired control DNA as the gold standard, we identified 136 microsatellite stable (MSS) and 36 microsatellite instability-high (MSI-H) colorectal tumors in the test group and 259 MSS and 47 MSI-H colorectal tumors in the validation group. Using the quasi-monomorphic variation range of each marker rather than paired normal DNA, the 5-marker panel identified all MSI-H colorectal tumors in the test and validation groups, when MSI-H was defined as ≥2 unstable markers. Our study demonstrates that the 5-marker panel within a multiplex polymerase chain reaction of the Promega MSI analysis kit accurately identifies all MSI-H and 95.2% MSS colorectal tumors without using paired normal DNA.