International Units Data for Cytokines
International Units (IU) are used to achieve comparability in the activity of cytokines in an internationally agreed manner. IU are calculated using a validated assay in which the cytokine of interest is tested side by side with the defined World Health Organization (WHO) standard obtained from the National Institute for Biological Standards and Control (NIBSC). The activity of the cytokine is then normalized using this standard.
STEMCELL is working toward providing IU data for all cytokines for which a WHO standard exists. To date, we have IU equivalence data for the following cytokines:
The specific activity (units/mg) of a cytokine is typically calculated using a bioassay with one set of conditions and parameters, and a defined cell line. Due to inherent variability, the specific activity measured with a different cell line, cultivated under different conditions in a different lab, can differ significantly. In order to achieve comparability for the specific activities, a defined standard set by the WHO needs to be used as a reference.
The activity of a reference standard is reported as the number of units in a given mass. Units are commonly referred to as Reference Units (RU); however, if the reference material has been tested internationally by the WHO, then it is designated as a WHO international standard. The units for these reference standards are International Units (IU).
For more information on IU conversion, including a sample calculation, read the Technical Tip: Biological Activity of Cytokines: Specific Activity (units/mg) vs. International Units.
Frequently Asked Questions
Is the conversion of EC50 to units/mg the same as International Units?
No. Conversion of EC50 to units/mg provides the specific activity, which is not the same as the International Unit. Specific tests need to be done against a WHO standard to obtain the IU data. It is not possible to make a calculation between the two without running the side-by-side comparison with a standard.
To learn how to calculate the specific activity (units/mg) from EC50, read the Technical Tip: Biological Activity of Cytokines: Specific Activity (units/mg) vs. International Units.
The bioassay used by STEMCELL to determine the activity is different to the bioassay for my application. Will the specific activity be the same in my application?
The numeric value for specific activity provided on the Product Information Sheet is specific to the assay described therein, using STEMCELL's standard conditions on the specified cell line. Thus, while the numeric value is a good demonstration of the basic functionality of the product, it does not automatically mean that this specific activity translates 1:1 to every application.
We advise all customers to use the given activity as a general benchmark, but to perform an additional assay adjusted for their specific application when needed. If you are unsure about the biological activity of a STEMCELL cytokine, please contact Product and Scientific Support.
STEMCELL Technologies has over 200 human, mouse, rat, and recombinant animal component-free (ACF) cytokines, chemokines, and growth factors in a variety of sizes, for various research applications. Explore cytokines for the following research areas: