What is a “unit” of endotoxin?

Initially endotoxin was measured in weight (nanograms).?It soon became apparent that a nanogram of one bacterial endotoxin could vary significantly from that of another according to the fine structure of the molecule (number of acyl groups and phosphates attached to lipid A, for example).?With this realization came the need to gauge endotoxin relative to its biological effect, typically referred to as its potency/activity.?At the time, the only measurable effect associated with endotoxin was fever in mammals (pyrogenicity).?The lowest amount of endotoxin that brings about fever defines the “threshold pyrogenic response” (TPR) level.?In man and rabbits this level was determined to be about 1 nanogram per kilogram when using a standardized process to create the endotoxin and a standard organism (E. coli).?Therefore, a “unit” of endotoxin (EU) is set to equal 1/5th the amount needed to bring about the TPR.?

5 EU/kg represents the TPR level used to determine the tolerance limit (TL) for drugs according to the formula TL =K/M where K is the constant (5 EU/kg) and M is the maximum human dose of a specific drug.?A large dose will necessitate that it contains a lower amount of endotoxin than a small dose.?Routes of administration other than an injection, such as intrathecal which allows only 0.2 EU/kg, may have different TPR constants (K).

Of course, rather than using a mammal (rabbit or human), today’s users employ LAL and thus the LAL also has to be made in such a way that its detection ability is standardized.?This ability to detect endotoxin is measured in the same endotoxin units (0.05 or 0.005 EU/mL, for example) and referred to as the lysate’s sensitivity.?This is also referred to as “lambda” and this value is used in calculations to determine how far out a drug solution may be diluted.?The more sensitive the lysate, the more detection power is available and thus the more it can be diluted to overcome interfering factors.

Users of recombinant Factor C (rFC) in lieu of LAL are fortunate in that the test is completely analogous.?Same standard, same units of measurement, same formulas, same sensitivity, etc. This “sameness” is due to the sameness of the rFC reagent biosensor, the active protein zymogen in LAL that detects endotoxin but in rFC is produced in a bioreactor rather than bled from a horseshoe crab.?What rFC doesn’t have is interference from non-endotoxin components such as beta-glucans and mannans (rFC doesn’t include Factor G).??

Long story short, a “unit” of endotoxin is 0.2 (1/5th) a nanogram of a specific standardized endotoxin.?Natural endotoxin types, or more typically a mixture of types in a solution, will give a detection reading that is relative to the activity of the standard endotoxin unit.?A solution that is a smorgasbord of bacteria can thus be measured in a way that is relevant to its activity rather than weight.??