Endotoxin in blood products: correlation between the Limulus assay and the rabbit pyrogen test

The possibility of replacing the rabbit pyrogen test by the Limulus (LAL) test, as a final release test for plasma products, was investigated. The LAL test measured the endotoxin content quantitatively, using a chromogenic substrate. The samples were boiled and centrifuged to remove inhibiting substances, which represent a major problem when testing plasma protein samples. In order to correlate the LAL test to the rabbit test, parallel assays were performed on 85 batches of two different human albumin preparations. For both products, a positive correlation was observed between the two tests. However, the pass/fail limit of the rabbit test was found at different endotoxin levels, corresponding to about 2 and 20 ng/kg, respectively. This discrepancy could be removed by extracting the endotoxin before administration to rabbits. It is concluded that the endotoxin, detected in plasma products by the LAL test, may be present in a non-pyrogenic state.