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Limulus Amebocyte Lysate (LAL) is an aqueous extract of blood cells (amebocytes) from the horseshoe crab, Limulus polyphemus. LAL reacts with bacterial endotoxin or lipopolysaccharide (LPS), which is a membrane component of Gram negative bacteria. This reaction is the basis of the LAL test, which is used for the detection and quantitation of bacterial endotoxins.

The LAL cascade is also triggered by (1,3)-β-D-glucan. Both bacterial endotoxins and (1,3)-β-D-glucan are considered "Pathogen-Associated Molecular Patterns", or PAMPS, substances which elicit inflammatory responses in mammalian systems.

There are three basic LAL test methodologies: gel-clot, turbidimetric, and chromogenic. The primary application for LAL is the testing of parenteral pharmaceuticals and medical devices that contact blood or cerebral spinal fluid. In the United States, the FDA has published a guideline for validation of the LAL test as an endotoxin test for such products [1]. Additional information about horseshoe crabs, the LAL test and the history of its development can be found at http://www.horseshoecrab.org/med/med.html.

One of the most time consuming aspects of endotoxin testing using LAL is pretreating samples to overcome assay inhibition and enhancement. Agents such as EDTA and heparin are known to affect the assay if they are present in sufficient concentrations. All assays, independent of methodology are standardized using endotoxin in water. Therefore, unless your sample is water, some components of the solution may interfere with the LAL test such that the recovery of endotoxin is affected. If the product being tested causes the endotoxin recovery to be less than expected, the product is inhibitory to the LAL test. Products which cause higher than expected values are enhancing. Overcoming the inhibition and enhancement properties of a product is required by the FDA as part of the validation of the LAL test for use in the final release testing of injectables and medical devices. Proper endotoxin recovery must be proven before LAL can be used to release product.

More information on overcoming Inhibition and enhancement can be found in the following article: Interference with the LAL Test and How to Address It