In terms of cytokine activity, what is the difference between International Units (IU) and specific activity?

International Units (IU) and specific activity are two different ways of measuring the biological potency of cytokines, but they serve different purposes and are calculated differently.

International Units (IU) are standardized measurements established by the World Health Organization (WHO) and the National Institute for Biological Standards and Control, UK (NIBSC) to provide a consistent way of comparing cytokine activity across different laboratories and products. They are determined by testing a cytokine and comparing it with an NIBSC protein with a known biological activity that acts as an internationally recognized reference standard under specific conditions. To calculate the IU of a cytokine, the cytokine must be assayed side by side with an NIBSC standard. This standardized process allows for more reliable comparisons between different cytokine preparations, regardless of their source or manufacturing process. CST offers cytokines and growth factors sold by weight and not IU. We are not assaying our cytokines against the NIBSC standards at this time.

Specific activity, usually expressed in units per milligram (units/mg), measures how much biological effect a cytokine has per unit of mass, and is a function of cytokine or growth factor purity and appropriate protein folding. It's typically calculated using a bioassay that determines the ED50—the concentration of cytokine needed to produce 50% of its maximum biological effect. The following formula can be used to convert the activity of an ED50 in ng/ml to specific activity in units/mg:
 

Specific activity (units/mg) = 1,000,000 / ED50 (ng/mL)

While specific activity is useful for assessing a cytokine's potency, it can vary between different bioassays and laboratory conditions, making direct comparisons between products or studies challenging.