Background <p>Colorectal cancer is one of the most common and deadly malignancies worldwide. Traditionally, all colon tumours have been treated as a single entity, but growing evidence shows that tumour behaviour differs markedly according to tumour sidedness. Right-sided and left-sided tumours exhibit distinct molecular profiles, prognoses and therapeutic responses. A liquid biopsy is a minimally invasive approach to monitor tumour dynamics in real time, offering valuable prognostic and predictive insights into metastatic colorectal cancer. However, despite increasing research on liquid biopsy-based biomarkers, the potential impact of tumour sidedness on their interpretation has not been systematically explored.</p> Methods <p>We evaluated circulating RAS mutational status, RAS mutant allele fraction and cell-free DNA concentration and fragmentation in plasma from 232 patients with metastatic colorectal cancer.</p> Results <p>Our findings reveal that tumour sidedness substantially modulates the prognostic value of these circulating biomarkers. In left-sided tumours, worse outcomes were associated with the presence of circulating RAS mutations, higher cell-free DNA concentrations and elevated carcinoembryonic antigen levels. In contrast, among right-sided tumours, only mutant allele fraction levels revealed significant prognostic relevance.</p> Conclusions <p>Our results demonstrate that tumour sidedness critically influences liquid biopsy interpretation in metastatic colorectal cancer, underscoring the need to incorporate the primary tumour location into the clinical assessment of circulating biomarkers.</p>