Depending on the source of the endotoxin, the conversion from endotoxin units to nanograms will vary. The FDA initially defined the Endotoxin Unit (EU) as the endotoxin activity of 0.2 ng of Reference Endotoxin Standard, EC-2 or 5 EU/ng. To convert the current FDA RSE, EC-6 from EU’s into ng, the conversion is 10 EU/ng. Previous versions of the BET chapter <85> in USP stated a suitable CSE has potency between 2 and 50 EU/ng.