Metastatic spread is a crucial process in which some questions remain unanswered. In this work, we focus on tumor cells circulating in the bloodstream, so-called Circulating Tumor Cells (CTCs). We aim to characterize their trajectories under the influence of hemodynamic forces and adhesion forces resulting from interaction with an endothelial layer using in vitro measurements performed with a microfluidic device. This essential step in tumor spread precedes intravascular arrest and metastatic extravasation. Our strategy is based on a differential equation model – a Poiseuille model for the fluid velocity and an ODE system for the cell adhesion model – and allows us to separate the two phenomena underlying cell motion: transport of the cell through the fluid and adhesion to the endothelial layer. A robust calibration procedure enables us to characterize the dynamics. Our strategy reveals the expected role of the glycoprotein CD44 compared to the integrin ITGB1 in the deceleration of CTCs and quantifies the strong impact of the fluid velocity in the protein binding.