Endotoxin Units (EU) are a measure of the activity of the endotoxin. Endotoxins differ in their biological activity or potency; the pyrogenicity or LAL reactivity of one endotoxin preparation may be very different from that of another of the same weight. Conversely, two endotoxin molecules may be different sizes and different weights but may have the same reactivity in an LAL test. The potency of an endotoxin determined with one LAL reagent lot may differ from that determined with another lot. Expressing endotoxin concentrations in EUs avoids the issues of different potencies of different endotoxins and allows us to compare results of different LAL tests performed in different labs. Consequently, we do not usually convert results of an LAL test in EU/ml to units of weight of endotoxin per ml.
References: Associates of Cape Cod, Inc. LAL Update, Vol. 15, No. 4, December 1997. Gould, M.J. 1998. Performing the LAL gel-clot test in facilities. Nephrology News & Issues 2(11):26-27,29.