e19327 Background: Micro-Satellite Instability (MSI) or DNA Mismatch Repair (MMR) is an established prognostic test associated with response to chemotherapies in Lynch Syndrome, colorectal cancer and other indications. Recently, MSI/MMR status has been shown to predict response to immune checkpoint inhibitors (CPI). Pembrolizumab has been approved in the USA for MSI-high or mismatch repair deficient patients for all indications. A similar approval has not been obtained in the EU. This data research project examined MSI/MMR testing in Europe and its possible association with use of CPI therapies. Methods: This study was conducted using Oncology Dynamics, an IQVIA oncology cross-sectional survey, collecting anonymized real-world patient-level data. 97,807 and 96,111 drug treated patients for solid tumors in 2018 & 2019, respectively, were collected across 5 European countries (France, Germany, Great Britain, Italy and Spain). Data included number of patients tested for MSI/MMR, results of the test, site and funding type, and the drug(s) prescribed. Results: European MSI/MMR test rates ranged from 3% in Italy in 2018 to 12% in France in 2019. Testing rates increased modestly in all countries over the period. Testing rates were determined in 25 specific indications plus all other indications. As expected, rates were highest in colorectal and stomach cancer. Testing was observed in other indications including Esophageal, Brain and Melanoma all with rates of 10% or higher. The percentage of MSI-High patients ranged from a low of 13% in Italy in 2019 to 26% in France in 2019. Finally, 17% of MSI-High patients are treated with CPIs compared to 5% of MSI-Low patients across all countries in 2019. Conclusions: The results over two years and almost 200,000 patients present insights into the use of MSI testing in 5 European countries. MSI testing is performed in many indications in addition to those typically associated with the biomarker. The increase in CPI use in MSI-High patients suggest that test results may impact use of checkpoint inhibitors although causation may not be inferred from the current analysis. However, the correlation suggests that further analysis is warranted.