Limulus Amebocyte Lysate (LAL) is a biological reagent prepared from the lysate of amebocytes extracted from the blue blood of the horseshoe crab, a marine arthropod. It is produced by low-temperature freeze-drying and is specifically used for the detection of bacterial endotoxins and β-D-glucans.

Classification by Source

There are currently four genera and species of horseshoe crabs worldwide. Only Tachypleus tridentatus Leach and Limulus polyphemus Linnaeus can be used to manufacture LAL reagents.

The corresponding reagents are Tachypleus Amebocyte Lysate (TAL), derived from Tachypleus tridentatus, and Limulus Amebocyte Lysate (LAL), derived from Limulus polyphemus. TAL and LAL have the same efficacy.

Classification by Test Method

Gel-Clot Method

The gel-clot method is based on the coagulation reaction between the LAL reagent and endotoxin and is used for the qualitative or semi-quantitative detection of endotoxins.

Common specifications of gel-clot LAL reagents include 0.1 mL/vial, 0.5 mL/vial, or larger fill volumes. Before use, the reagent should be reconstituted with pyrogen-free water (Water for Bacterial Endotoxins Test).

The gel-clot method uses the presence or absence of gel formation as the reaction endpoint. This method is simple and economical, does not require specialized analytical equipment, and can be used for qualitative or semi-quantitative endotoxin testing.

Specific LAL Reagent

The Specific LAL Reagent, also known as a Factor G-free LAL Reagent, is a product developed domestically for specific endotoxin detection. It reacts specifically with endotoxins and avoids interference from the Factor G pathway, resulting in more reliable test results. It is an ideal reagent for applications in pharmaceutical quality control and clinical testing.

Turbidimetric Methods

The kinetic turbidimetric method and endpoint turbidimetric method are both classified as turbidimetric methods. The turbidimetric method determines the endotoxin concentration by measuring changes in turbidity during the reaction between the LAL reagent and endotoxin.

The endpoint turbidimetric method has not been commercialized. The kinetic turbidimetric method, also known as the kinetic nephelometric method, measures either the reaction time required for the turbidity of the reaction mixture to reach a predetermined absorbance value or the rate of increase in turbidity.

Chromogenic Methods

The endpoint chromogenic method and kinetic chromogenic method are both classified as chromogenic substrate methods.

The chromogenic substrate method determines the endotoxin concentration by measuring the amount of chromophore released when a specific substrate is cleaved by the coagulation enzyme generated during the reaction between the LAL reagent and endotoxin. The endotoxin concentration is determined based on the color intensity of the reaction product. This method is also known as the colorimetric method.

In 2005, Xiamen LAL Reagent Experimental Factory Co., Ltd. introduced an LAL reagent kit using LAL tetrapeptide as the chromogenic substrate. The product offers strong resistance to interference and a sensitivity of up to 0.005 EU/mL, with quality reaching internationally advanced levels.

LAL Reagent Selection Guide

If you only need to determine whether the endotoxin level of a sample meets its specified endotoxin limit:
The gel-clot LAL reagent can be selected. By determining the endotoxin limit and Maximum Valid Dilution (MVD) of the test product, an interference test can be performed to determine the appropriate sensitivity of the LAL reagent to be used.

If you need to quantitatively determine the endotoxin concentration in a sample:
A chromogenic LAL reagent kit or kinetic turbidimetric LAL reagent should be selected.

The kinetic turbidimetric method and kinetic chromogenic method require a kinetic photometric instrument equipped with an incubation system and corresponding software, such as the ELx808 Microplate LAL Reader with IU and TALgent software.

The kinetic turbidimetric and kinetic chromogenic methods are simple and convenient, can be completed in a single step, and provide a wide linear range.

The endpoint chromogenic method requires a microplate reader or visible spectrophotometer for detection. A microplate reader equipped with an incubation system, such as the ELx808 Microplate LAL Reader, can help reduce reagent consumption.