Endotoxin testing: LAL, rFC and MAT
Endotoxin is a fragment of a dead bacterium. It survives autoclaving, it passes through a sterilizing filter, and the three tests that detect it do not measure quite the same thing.
What the contaminant is
Endotoxin is lipopolysaccharide, a structural component of the outer membrane of gram-negative bacteria. It is released when those cells die, so killing the bacteria is exactly what sets it loose.
Two properties make it the contaminant it is. It is heat stable, so autoclaving destroys the organism and leaves the endotoxin behind. And the molecule is small enough to pass a 0.22 micron sterilizing filter without difficulty. A solution can be genuinely sterile and heavily pyrogenic at the same time, and neither of the two standard sterilization methods addresses it.
The three tests
LAL
Limulus amebocyte lysate, from horseshoe crab blood. Three formats: gel clot, which is a yes-or-no at a threshold; turbidimetric, which times the onset of cloudiness; and chromogenic, which releases a colored product and is read on a plate reader. Chromogenic is the usual choice where a number is wanted.
rFC
Recombinant Factor C, the first enzyme of the same cascade, produced without animals. Two advantages beyond supply: it responds only to endotoxin, so beta-glucans from cellulose filters and plant material do not trigger the false positives they cause in LAL, and lot-to-lot variability is lower.
MAT
Monocyte activation test. Human cells are exposed to the sample and the cytokine response is measured. It is the only one of the three that detects non-endotoxin pyrogens, gram-positive cell wall material among them, because it measures the biological response rather than one molecule.
How results are reported
- Unit
- EU, endotoxin unit. Roughly 0.1 to 0.2 ng of reference LPS per EU.
- Reported as
- EU per mL for a solution, or EU per mg for a solid.
- Typical gel clot sensitivity
- 0.03 to 0.25 EU/mL depending on the lysate lot
- Chromogenic range
- About 0.005 to 50 EU/mL
- What must accompany a result
- The method, the sensitivity, the dilution factor and the positive product control recovery.
Endotoxin units are defined against an international reference standard rather than by mass, because potency varies by bacterial source.
Masking, and why a clean result can be wrong
Low endotoxin recovery is a documented and specific failure. In certain combinations, typically a chelating buffer such as citrate or phosphate together with a non-ionic surfactant such as polysorbate, endotoxin present in the sample becomes undetectable to the assay over a period of hours to days. The LPS is still there. The assay reports nothing.
This is why a positive product control matters more than the sample result. Spiking a known quantity of endotoxin into the actual sample matrix and confirming it is recovered within about 50 to 200 percent demonstrates that the assay could have seen endotoxin in that matrix. Without it, a clean number is compatible with both a clean sample and a blind assay.
Where it enters, in order of likelihood
- 01
Water
The largest single source. Ordinary purified water supports gram-negative growth, and the biofilm in a poorly maintained purification loop sheds endotoxin continuously. Water for injection grade exists for this reason.
- 02
Glassware and containers
Washing removes organisms and leaves endotoxin. Glass is depyrogenated by dry heat, typically 250 degrees for thirty minutes or more, which is a different process from sterilization and achieves a different thing.
- 03
Raw materials
Anything of biological origin, and anything that has been in contact with water upstream. It arrives with whatever history it had.
- 04
Handling
Open manipulations introduce it. A closed process with fewer transfer steps has fewer opportunities.
Questions
Common questions
Does filtering a solution remove endotoxin?
No. A 0.22 micron filter removes bacteria and passes lipopolysaccharide freely. Removal requires a different mechanism: anion exchange resin, ultrafiltration below the aggregate size, or a filter specifically designed to bind endotoxin.
Does autoclaving destroy endotoxin?
No. It is heat stable at autoclave conditions, and autoclaving kills gram-negative cells, which releases more of it. Depyrogenation of glass takes dry heat at around 250 degrees, well above autoclave temperature.
Is rFC accepted in place of LAL?
It is recognized in the European Pharmacopoeia as an alternative and has a chapter in the United States Pharmacopeia. For research material the question is mostly academic, since what matters is that a named method, a sensitivity and a positive product control recovery appear alongside the number.
What is a reasonable endotoxin specification?
It depends entirely on the intended use and the exposure route, so a supplier quoting one limit for everything is quoting a number with no context. What a certificate can usefully state is the measured value, the method, the dilution factor and the control recovery, and let the reader apply their own limit.
Cite as: SBz Industries research library, "Endotoxin testing: LAL, rFC and MAT", https://www.sbzindustries.com/research/endotoxin-testing, updated 2026-09-11.
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