How do you measure cytokine activity

A cytokine activity is measured by its ability to function within a cellular environment. The function of a cytokine varies between cytokines and the effects measured can be proliferation,chemotaxis,viral resistance, secretion of cytokines…etc.
The parameter to describe cytokine activity is the ED50 (half maximum Effective Dosage), a measure of the required concentration needed to achieve 50% maximal response.
1 Unit of activity is defined as the ED50 value.
While this is a wholly relative way to quantify activity, dependent on how each activity assay was performed, the WHO does provide standardization to cytokine activity via reference materials with established assays and units of activity IU(international units).