Flow Cytometer

Peter Erhardt, Ph.D., Scientist

Flow cytometry (FCM) is a method for quantifying the types of cells in a mixture.  It may be used simply for cell analysis or it can be used for cell sorting, where the subpopulations are physically separated from each other and used for further experiments.  No other method allows as a rapid a quantitative and detailed analysis of subpopulations of cells as FCM.  It is popular for both research and clinical diagnosis, because it can quickly and inexpensively analyze a large number of samples (30-50 samples/hour), much faster than the best hematologist.

To do the analysis, fluorescently labeled live or fixed cells in single-cell suspension are passed single-file as a fine stream through a laser beam.  Each cell scatters some of the laser light and also emits fluorescent light excited by the laser. The cytometer typically measures several parameters simultaneously for each cell: a) the low-angle forward scatter intensity, approximately proportional to the cell diameter; b) the orthogonal (at 90º angle) scatter intensity, approximately proportional to the quantity of granular structures within the cell;  and c) fluorescence intensities at 3-4 wavelengths. The computer records the data and displays them graphically.

The use of flow cytometers has been increasing since they became commercially available in the early 1970s. There are about 7,000 flow cytometers in use worldwide. In research, fluorescent antibodies are often used to measure specific cell components such as surface receptors, expression of a transgene or binding of viruses, or hormones to their receptors. Another typical measurement is the amount of total DNA per cell for analyzing the growth and death of cells. FCM also has important clinical applications in diagnosis, prognosis, and optimization of therapy. For example, FCM can distinguish subtle changes in the types of blood cells to discriminate between different types of blood cell cancers (e.g. leukemias and lymphomas).