Comparison of the Idylla™ MSI assay with the Promega™ MSI Analysis System and immunohistochemistry on formalin-fixed paraffin-embedded tissue of endometrial carcinoma: results from an international, multicenter study.

Determination of microsatellite instability (MSI) and mismatch repair deficiency (MMRD), respectively, in endometrial carcinomas (ECs) is important for diagnostic and prognostic purposes, identification of Lynch syndrome carriers, and selection of patients for immunotherapy. The Idylla™ MSI assay is fully automated, does not require non-tumoral tissue, and can be performed in about 150 min. Two hundred forty-two formalin-fixed paraffin-embedded (FFPE) EC samples from 7 international centers were tested by the Idylla™ MSI assay and compared to the Promega™ MSI Analysis System and immunohistochemistry (IHC) for MMR proteins. The cases were selected with an enrichment of MSI EC to around 40%. Concordance was 87.5% between the Idylla™ MSI assay and IHC and 88.58% between IHC and Promega™ MSI assay. Concordance between Idylla™ and Promega™ MSI assays was 89.91%. Discordant results occurred more frequently in cases with MSH6 or PMS2 deficiency. Invalid cases occurred with the three techniques (IHC, 7.00%; Promega™ MSI assay, 5.37%; and Idylla™ MSI assay, 2.47%). The concordance rate between Idylla™ MSI assay and the other 2 methods increased to 88.83% for IHC and to 91.22% for the Promega™ MSI assay when the cutoff of instability in the scoring system was moved from 0.5 to 0.3. The Idylla™ MSI assay is a rapid and highly concordant test for MSI in EC. Modification of the Idylla™ scoring system could increase the sensitivity and specificity of the MSI assay for EC analysis.