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Background: We investigate the current knowledge on circulating tumour DNA (ctDNA) and its clinical utility in predicting outcomes in patients with metastatic colorectal cancer (mCRC)., Methods: PubMed, Embase, Cochrane Database of Systematic Reviews and Cochrane Central Register of Controlled Trials were searched. Last search 16/12/2020. We included studies on patients with mCRC reporting the predictive or prognostic value of ctDNA. We performed separate random-effects meta-analyses to investigate if baseline ctDNA and early changes in ctDNA levels during treatment were associated with survival. The risk of bias was assessed according to the Quality in Prognosis Studies tool., Results: Seventy-one studies were included with 6930 patients. Twenty-four studies were included in meta-analyses. High baseline ctDNA level was associated with short progression-free survival (PFS) (HR = 2.2; 95% CI 1.8-2.8; n = 509) and overall survival (OS) (HR = 2.4; 95% CI 1.9-3.1; n = 1336). A small or no early decrease in ctDNA levels during treatment was associated with short PFS (HR = 3.0; 95% CI 2.2-4.2; n = 479) and OS (HR = 2.8; 95% CI 2.1-3.9; n = 583). Results on clonal evolution and lead-time were inconsistent. A majority of included studies (n = 50/71) had high risk of bias in at least one domain., Conclusions: Plasma ctDNA is a strong prognostic biomarker in mCRC. However, true clinical utility is lacking., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)