I do not have any experience in this type of assay, but I think you can adapt a cell-based ELISA protocol:http://www.biotechniques.com/Posters/Ce ... 03227.html
Since the protein of interest is on cell surface, you can skip the permeabilization step. Fixing the cells is important, or you will lose some cells during the washing steps.
Flow cytometry will be too time consuming, if you have a large number of samples.
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