485 Background: Circulating tumor cells (CTC) appear to be associated with poor prognosis in patients with metastatic colorectal cancer (mCRC). ScreenCell technology, which is based on the separation of CTCs from other circulating cells by blood filtration, could allow clinical use of this new biomarker. The aim of our study was to demonstrate the feasibility of CTC filtration in a clinical situation and to compare CTC cytological/molecular characterizations. Methods: Sampling of venous blood was performed in 39 patients with mCRC before starting chemotherapy regimen. For cytology, blood were filtered using ScreenCell Cyto devices. After staining with hematoxylin, each filter was analyzed by a cytopathologist. Cells visualized on the filter were considered as tumoral if they met the following morphological criteria: nuclear diameter >7micron, anisocytosis, membrane irregularities, presence of large nucleolus. For molecular biology, blood were filtered using ScreenCell MB columns: DNA was then extracted for KRAS genotyping. Prior to the clinical study, HCT116 cells were used in order to establish biological procedures. Results: 3 samples were not analysed due to blood coagulation. The presence of CTC was identified in 23/36 patients with mCRC (64%). KRAS mutation was present in 40% of tumor patients. In those patients, no mutation was observed on CTC specimens even when using whole gene amplification and PNA selective PCR, whereas KRAS mutation was detected in 10 HCT116 cells diluted in 3ml of normal blood using this same procedure. Conclusions: ScreenCell Technology allows cytological studies which appear to be fast and efficient for the detection of CTC in mCRC patients. Detection of CTC molecular abnormalities is a major challenge: implementation of molecular tests such as KRAS genotyping, allowing tumor characterization directly from minimally invasive blood sampling, could have tremendous clinical applications. Our first in vitro results highlight the fact that detection of somatic molecular alterations is feasible, from just a few tumoral cells diluted in blood, which encourages further developments using other molecular approaches.