Microsatellite instability diagnosis using the fully automated Idylla platform: feasibility study of an in-house rapid molecular testing ancillary to immunohistochemistry in pathology laboratories

Aim To study the performances of the Idylla MSI Assay in the diagnosis of microsatellite instability (MSI) or microsatellite stability (MSS). Methods We selected 12 tumour samples previously tested for MSI focusing on cases with discrepant results between MLH1, PMS2, MSH2 and MSH6 immunohistochemistry and microsatellite molecular analyses (five cases) or doubtful immunohistochemistry (two cases). Idylla MSI Assay was compared with retrospective immunohistochemistry and molecular results. Results Idylla MSI Assay showed an almost perfect concordance with microsatellite analysis results previously obtained (only one case with not fully conclusive analysis due to sample exhaustion). The full molecular analysis took less than 150 min per sample and revealed no mutation in any of the seven microsatellite sequences in five MSS samples and four to six mutated ones in seven MSI-High samples. Conclusion At the era when the determination of MSI/MSS status is becoming important for rapid treatment choices, the Idylla MSI Assay consists of a valuable easy-to-perform diagnostic tool that allows, complementary to MLH1, PMS2, MSH2 and MSH6 immunohistochemistry, the diagnosis of MSI/MSS status in a single day.