Validation of viable and apoptotic CEC enumeration. Representative spiking of peripheral blood with human umbilical vein endothelial cells (HUVECs) used to validate CEC enumeration by flow cytometry (top panels), and evaluation of apoptosis by 7AAD in HUVECs exposed to FAS ligand (bottom panels). Top panels show the gate used to exclude platelets, dead cells, and debris (left panel), and enumeration of HUVECs depicted as CD45^–, P1H12⁺ cells (right panel). The detection limit of our procedure was 0.1 cells/μL, and specificity was more than 90%. Repeated CEC measurements in blood samples from more than 30 healthy subjects over time have shown a less than 14% variability. Panels of the bottom show representative 7AAD staining of HUVECs before (left) and after (right) exposure to FAS ligand.