e16302 Background: Approximately 80% of pancreatic ductal adenocarcinoma (PDAC) patients present with advanced disease at diagnosis, with a 5-year survival rate of less than 5%. According to molecular profiling, mutations in KRAS have been described in 90% of PDAC patients, and correlated with poor prognosis. Nevertheless, the invasiveness of tumor biopsy and the difficulties in obtaining quality samples in some patients remains a concern. Circulating free DNA (cfDNA) is considered a surrogate marker of tumor burden and may also act as a prognostic factor that can be used independently from tumor biopsy. In this study, we aim to explore molecular prognostic biomarkers using plasma samples. Methods: Patients with metastatic PDAC harboring KRAS mutation in tissue samples and treated with first-line chemotherapy were identified at Vall d’Hebron University Hospital. Clinical features and plasma samples were collected before starting any treatment. DNA mutations were analyzed in plasma and compared with tissue samples. In plasma, detection of KRAS mutation and total cfDNA concentration were evaluated for clinical significance. A cfDNA minimum 100 genome equivalent threshold was proposed based on results and utility. A Chi-square test was planned to prove the independence between cfDNA levels and KRAS detection in plasma. Survival analysis was performed using the Kaplan-Meier method. Results: We included 33 PDAC patients with a median age of 66.1 years old, 57% were women. All of them were treated with gemcitabine and Nab-paclitaxel. At the final analysis cut-off, 31 patients had died . KRAS mutation was detected in 25 patients’ blood samples (75.8%) and cfDNA-High levels were considered for 20 patients (60.6%). 19 of 20 (95%) patients with cfDNA-High correlated with KRAS blood detection, showing significant dependence (p-value 0.001). Median overall survival (OS) was 10.5 months in the KRAS detected group compared with 22.6 months in the KRAS non-detected group (HR for death 4.70; confidence interval (CI) 1.70-12.98, p-value 0.002). Better separation was seen using cfDNA where median OS was 9.8 months in the cfDNA-High group compared with 21.0 months in the cfDNA-Low group (HR for death 5.38; CI 2.17-13.31, p-value < 0.001). Conclusions: Liquid biopsy can provide quality prognostic information in metastatic PDAC patients. On account of this, the cfDNA level is proposed as a prognostic factor. However, prospective cohorts to validate this information are needed.[Table: see text]