The gel clot LAL assay is the simplest form of the LAL assay and most closely resembles the originally developed BET method. It provides an easy to read qualitative result indicating the presence or absence of endotoxin in the test sample. It is most often mentioned as the official referee test in pharmacopeial monographs.
The customary gel clot method involves mixing the LAL reagent with the test sample in a tube, which is then incubated. If endotoxin is present in the sample, the clotting response by the LAL reagent will be initiated.After the incubation period, the tube is examined by eye to determine whether a clot (or gel) has formed at the bottom of the tube. A positive result is indicated by the presence of a clot which stays at the bottom of the tube upon gentle inversion, whereas a negative result is indicated by liquid flowing down the inside of the tube.