Comparison of 3 PCR-based assays for microsatellite instability detection in formalin-fixed paraffin-embedded tissues of patients with colorectal cancer.

e15042 Background: Routine assessment of microsatellite instability (MSI) status in colorectal cancer (CRC) patients can help with initial screening for Lynch syndrome, prediction of tumor prognosis and selection of patients for immunotherapy. Immunohistochemistry (IHC) and PCR-based approaches represent the current clinical laboratory standard for the evaluation of MSI status. Both Bethesda and Promega marker panels have been used widely in the world and the Bethesda panel and the six mononucleotides panel (NR21/BAT25/NR24/NR27/MONO27/BAT26) have been approved by the National Medical Products Administration (NMPA) in China. The Idylla MSI Test, a fully automated system based on real-time PCR, which incorporates 7 novel MSI loci (ACVR2A/BTBD7/DIDO1/MRE11/RYR3/SEC31A/SULF2) can perform MSI analysis within 150min using 1-3 tissue slides. The aim of this study was to evaluate the performance of the three 3 PCR-based assays for MSI detection including Idylla MSI Test, Bethesda and that six mononucleotides panel. Methods: One hundred and fifty-three formalin-fixed paraffin-embedded (FFPE) tumor tissues from patients with CRC were collected and their MSI status were analyzed using 3 PCR-based assays, respectively, i.e. the Idylla MSI, “2B3D” NCI panel and the six mononucleotides panel. All samples were previously evaluated by IHC method. Results: In the all 153 patients, IHC identified 27 cases to be MMR-deficient (dMMR), and 126 to be MMR-proficient (pMMR). Using IHC as a reference, the Idylla MSI had a higher consistency that showed overall agreement, sensitivity and specificity as 93.46%, 92.89% and 93.65%, respectively, while the corresponding values of the six mononucleotides panel were 84.31%, 92.59% and 82.54% respectively, and the “2B3D” NCI panel showed a sensitivity of 92.16%, a specificity of 88.89% and an overall concordance of 92.86%. The PCR-based approaches identified 11 more MSI-H cases than IHC while 22 dMMR cases identified by IHC were detected by all the 3 assays, 2 dMMR by 2 assays, 1 dMMR by one assay only and 1 dMMR by neither assays above. Moreover, the Idylla MSI showed advantages in ease of use, turnaround time (TAT) and small dose of tissue input. Conclusions: Our findings support that the Idylla MSI assay provides a rapid, reliable, and ease-to-use solution to MSI detection in Chinese CRC patients with high sensitivity and specificity. The inconsistent cases between IHC and PCR-based approaches need a further analysis.