Circulating endothelial cells (CECs) are a reliable biomarker for cardiovascular diseases (CVDs). A major unresolved challenge limiting the widespread use of CECs for the diagnosis and monitoring of CVDs is their unreliable detection. This problem is mainly attributed to the low sample volume (5-10 mL) of commonly used ex vivo CEC isolation methods. To overcome this limitation, the BMProbe for the in vivo isolation of CECs is proposed. It consists of a twisted medical flat wire with a polymer-coated surface functionalized with anti-CD105 antibodies. A combined experimental and numerical study is performed to investigate which flow conditions lead to an increased cell attachment to the probe's surface. Endothelial cells are solved in a dextran solution and circulated in a flow system containing the BMProbes. Microscopic images of the attached CECs are taken. In addition, the experiments are simulated using a computational fluid dynamics (CFD) flow solver to quantify the flow conditions at the probe's surface. The microscopic images are superimposed with the CFD data to investigate the influence of wall shear rate and wall normal rate on the attachment of CECs to the probe. Most of all attached cells (85.5%) are found in areas of negative wall normal rate.