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Aim: To determine the concordance between plasma and tissue RAS mutation status in metastatic colorectal cancer patients to gauge whether blood-based testing is a viable alternative. We also evaluated the change in mutation status on progression. Materials/methods:RAS testing was performed on plasma from patients commencing first-line therapy (OncoBEAM™ RAS CEIVD kit). Results were then compared with formalin-fixed paraffin embedded tumor samples. Results: The overall percentage agreement (concordance) was 86.0% (86/100), which demonstrates that blood-based testing is an alternative to tissue-based testing. Reproducibility was 100% between three laboratories and 20% showed changes in their RAS mutational status on progression. Conclusion: These results show good concordance between tissue and plasma samples and suggest the need for longitudinal plasma testing during treatment to guide management decisions. [ABSTRACT FROM AUTHOR]