Usually, a cell that is secreting the protein of interest is isolated using an antibody-antibody complex that coats the cell and is able to "catch" the secreted molecules.
The FACS method is broadly similar to the enzyme-linked immunosorbent assay (ELISA) antibody format, except that the encapsulated cells remain intact.
Further advances now mean that it is possible to extract the secreting cells using a magnetic-based separation system or using a flow cytometer.
A number of commercial applications exist for secretion assay.
One such example is the Gel Microdrop (GMD) technology, developed by One Cell Systems.