The detection of endotoxin by in vitro production of endogenous pyrogen: Comparison with limulus amebocyte lysate gelation

The sensitivities of leukocyte endogenous pyrogen (EP) production and limulus amebocyte lysate (LAL) gelation to endotoxin from E. coli (minimum i.v. pyrogenic dose 4 ng/kg in rabbits) were determined. Concentrations of 0.5–1.0 ng/ml could be detected by LAL. The minimum endotoxin concentration which generated detectable EP from 2 × 10⁶ monocytes was 10-fold lower (0.05–0.1 ng/ml). At an endotoxin concentration of 0.4 ng/ml the minimum number of monocytes required for detectable EP production was 5 × 10⁵. It is concluded that the LAL gelation test cannot safely be used to exclude significant endotoxin contamination in a cellular system where EP production is being measured. The same conclusion applies even more forcibly to the in vitro production of lymphocyte activating factor (LAF, interleukin-1), since it appears that LAF and EP are identical and sub-pyrogenic amounts of EP are easily detectable in the LAF assay.