assay flow cytometry assay flow cytometry assay A cytometry assay in which an input cell population is put in solution, is passed by a laser, and optical sensors are used to detect scattering of the laser light and/or fluorescence of specific markers to count and characterize the particles in solution. IEDB IEDB Using a flow cytometer to quantitate the percent of CD3 positive cells in a population by labeling them with a FITC tagged anti-CD3 antibody. FACS