Endotoxin is lipopolysaccharide, a structural component of the outer membrane of Gram-negative bacteria. It is released when those cells die and break apart, and it is heat-stable to a degree that ordinary sterilisation does not destroy. A lot can therefore pass a sterility test and still carry a significant endotoxin burden, because killing the organism does not remove the fragment it leaves behind.
That is the whole reason the assay exists as a separate line on a certificate.
What the LAL assay is
The limulus amebocyte lysate assay uses a clotting cascade from the blood cells of the horseshoe crab, which responds to endotoxin with great sensitivity. The lysate is combined with the sample and the resulting reaction is measured. It is the basis of the compendial bacterial endotoxins test described in United States Pharmacopeia general chapter ⟨85⟩.
A recombinant factor C assay, usually written rFC, reproduces the first enzyme of that cascade without the animal-derived lysate. It has been gaining acceptance as an alternative method and removes the supply-chain and conservation questions attached to horseshoe crab harvesting. A laboratory using rFC should say so, since it is an alternative rather than the default compendial route.
The three method formats
- Gel-clot. The simplest format. Sample and lysate are incubated and the tube is inverted; a firm clot that holds indicates endotoxin at or above the lysate's labelled sensitivity. The result is a limit test, reporting pass or fail against a threshold rather than a continuous value.
- Turbidimetric. Measures the increase in turbidity as the cascade proceeds. Time to reach a set turbidity is proportional to endotoxin concentration, which yields a quantitative figure against a standard curve.
- Chromogenic. Uses a synthetic substrate that releases a coloured product when cleaved by the activated enzyme. Absorbance is read on a plate reader and quantified against a standard curve. This is the most common quantitative format in contract laboratories.
Reading an EU/mg figure
Endotoxin is reported in endotoxin units, abbreviated EU, defined against an international reference standard rather than as a mass. For a solid material the result is normalised to sample mass and expressed as EU per milligram.
A certificate line therefore has three parts worth reading separately: the measured value, the specification it is compared against, and the sensitivity of the method used. A result of "< 0.05 EU/mg" is only as meaningful as the assay's ability to detect below that figure. If the lysate sensitivity is above the specification, the test cannot demonstrate conformance no matter what it reports.
Interference, and the controls that catch it
Peptide samples can interfere with the cascade in both directions. Some matrices inhibit the reaction and produce a falsely low result; others enhance it and produce a falsely high one. Two controls address this and both should be stated.
- The positive product control spikes a known quantity of endotoxin into the sample itself. If the assay recovers that spike within an acceptable range, typically 50% to 200%, the matrix is not meaningfully interfering.
- The maximum valid dilution establishes how far the sample may be diluted to overcome interference while remaining able to detect the specification limit. Diluting past that point makes a passing result meaningless.
A certificate that reports a value without a positive product control is reporting a number the method has not shown it can trust in that specific matrix.
Why depyrogenation differs from sterilisation
Sterilisation kills organisms. Depyrogenation destroys or removes the endotoxin they leave behind, and it takes considerably harsher conditions, typically dry heat at several hundred degrees for a sustained period, or removal by ultrafiltration. Glassware and equipment in contact with material intended for parenteral research use are usually depyrogenated for this reason.
The practical consequence for a buyer is that the two certificate lines are not redundant. Sterility and endotoxin describe different hazards and different process controls.
What to ask for
- The method format: gel-clot, turbidimetric or chromogenic, and whether LAL or rFC.
- The lysate sensitivity and the maximum valid dilution applied.
- The positive product control recovery for that lot.
- The specification the result is being judged against, stated in the same units.
Endotoxin is frequently the line missing from a research-grade panel, because it costs money and most work does not strictly require it. What a certificate does not test for places that absence alongside the others, and sterility testing versus sterile filtration separates it from the sterility question it is often confused with.
This sits inside the wider picture of what gets tested and why, which why peptide testing matters sets out across the whole analytical panel.