The CellSearch® system uses a glass-surfaced cartridge. Immunomagnetically labelled cells expressing EpCAM are fluorescently stained and pulled to the surface of the chamber using a targeted magnetic field. a | The semiautomated imaging system captures four fluorescent images at each of 175 locations in order to create a 4-composite photograph of the entire surface of the chamber. b | A close-up of approximately 25 of the 175 locations, showing a large number of fluorescently labelled events present within each location. c | The CellTracks® software analyzes individual events in the image frames, creating a gallery of images including only those events where DAPI and cytokeratin fluorescence are co-located. The user examines these events for the presence of DAPI (purple), cytokeratin (green), and the absence of CD-45 (red) fluorescence. Events that meet the criteria of a CTC assay based on morphology, an intact nucleus, and cytokeratin expression, and which are negative for CD-45 expression (events boxed in red), are only a proportion of cytokeratin-positive events in the chamber. Other events (not boxed) that do not meet these criteria are not scored as CTCs. Permission obtained from Robert McCormack, Veridex, LLC. Abbreviations: CTC, circulating tumour cell; DAPI, 4′,6-diamidino-2-phenylindole. (Image courtesy of Robert McCormack, Veridex, LLC.)