For this reason R&D Systems typically refers to proteins by their mass and activity by ED 50. The method used, such as comparison with a WHO standard, must be the same between sources for a unit comparison to be valid. Different suppliers may have their own method for defining a unit. One must be careful when comparing units of activity between sources. 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). 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 be International units (IU). Typically, units are referred to as reference units (RU). The activity of the protein reference standard is indicated by the number of units contained in a given mass of material. 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. A unit is defined according to the standard by which it is compared.
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