Definition
The Limulus amebocyte lysate assay uses an extract of blood cells from the Atlantic horseshoe crab, whose clotting cascade is triggered by bacterial endotoxin. Three formats are common: the gel-clot method, which gives a pass or fail result at a set sensitivity; the turbidimetric method, which tracks cloudiness; and the chromogenic method, which measures colour released from a synthetic substrate. Results are reported as EU/mg. Peptides can interfere with the enzyme cascade, so validated assays include spike-recovery controls. Recombinant factor C assays, which avoid harvesting horseshoe crab blood, are an accepted alternative in several pharmacopoeias. The method is described in USP chapter 85 and its European and Japanese equivalents.