27 January 2021 Circulating tumor cells (CTC) are rare cells shed from a solid tumor or metastases that circulate through the bloodstream. Detection of CTC poses a challenge because they are present at extremely low concentrations, as few as a couple CTC in several milliliters of blood, which contain up to 3 to 6 million cells per mL. To determine the sensitivity and accuracy of rare cell detection on the Agilent NovoCyte Quanteon flow cytometer, a tumor cell spiking assay was used in which a small number of cells from a tumor cell line was added to whole blood and detected by a cell surface stain.