Abstract: Objective According to the revised Bethesda guidelines, the relationship between clinicopathological features and microsatellite instability (MSI) testing in nonfamilial colorectal cancers patients was analyzed, Our aim was to identify clinicopathological features that identify high MSI (MSI-H).Methods We enrolled 150 patients, standard microsatellite loci (BAT25、BAT26、D2S123、D5S346、D17S250) were amplified by polymerase chain reaction（PCR）with fluorescent primers, and the PCR products were analyzed by GeneMapper software; age at diagnosis, gender and site, were obtained; various pathological features(tumor grade, mucinous differentiation, histologic heterogeneity, Crohn's-like response) were observed by light microscope; the express of tumor infiltrating lymphocytes(CD4+ and CD8+) was detected by immunohistochemistry. Using a stepwise logistic regression model, a formula was generated that could be used to calculate the probability of a colorectal carcinoma being MSI-H based on pathological features. Results Among 150 cancers, MSI-H was 13.33％;Independent identifier were poor differentiation, histologic heterogeneity , Crohn's-like reaction and tumor-infiltrating lymphocytes, logistic regression formula has a sensitivity of 70.0% and a specificity of 99.2% and a accurate ratio of 95.3% for MSI-H. Conclusions MSI-H phenotype cancer is a type of nonfamilial colorectal cancer with specific pathological features, Clinicopathological features can identify efficiently MSI-H colorectal cancers.

Key words: microsatellite Instability(MSI), hereditary nonpolyposis colorectal cancer(HNPCC), colorectal carcinoma, clinicopathological, tumor infiltrating lymphocytes (TILs)