A unit is defined according to the standard by which it is compared. For instance, the World Health Organization (WHO) offers protein reference standards that are accessible to everyone and allow for the normalization of activity definitions across proteins from different sources. These are available fromNIBSC.

The activity of the protein reference standard is indicated by the number of units contained in a given mass of material. Typically, units are referred to as reference units (RU). When the reference material has been tested successfully in international, multi-center, collaborative studies it will be designated a WHO international standard, the units will then beInternational units (IU).

If there is a protein reference standard available, R&D Systems compares the activity of the reference standard with our protein, allowing for the conversion of mass into units (see table below).

Are all units the same?

No. One must be careful when comparing units of activity between sources. Different suppliers may have their own method for defining a unit. The method used, such as comparison with a WHO standard, must be the same between sources for a unit comparison to be valid. For this reason R&D Systems typically refers to proteins by their mass and activity by ED₅₀.

Sample Conversion R&D Systems Recombinant Human BMP-2 to Equivalent Units

To calculate the approximate mass/concentration of an R&D Systems cytokine in units, multiply the mass/concentration (in µg) by the ‘R&D Systems Cytokine 1 µg = units’ column in the chart, the second column from the right. For instance, a 10 µg vial of rhBMP-2, catalog #355-BM, would contain 7,810 units (10 times 781).

* In side by side evaluation of this standard with R&D Systems product, the standard demonstrated much lower maximal signal and thus cannot be compared using ED₅₀

**The reference material was obtained through the NIH Biodefense and Emerging Infections Research Resources Repository, NIAID, NIH.