Previously, I briefly discussed ELISA assays. Out of the ELISA method came the more sophisticated ELISPOT technique, which is a versatile tool for analyzing the substances secreted by peripheral blood and lymphoid cell populations. It is also extremely sensitive, capable of achieving single-cell resolution if properly applied. The ELISPOT technique allows researchers to measure the frequencies (i.e. the relative concentrations) of individual cytokine-, antibody- or granzyme-secreting cells within cultured cell populations.
Here we see how an ELISPOT assay is performed. That link also provides information on reagents for performing a Granzyme B assay.