rFC vs LAL endotoxin testing: what to check before switching

How do LAL, rFC and rCR differ inside the reagent?

Endotoxin testing recreates in a test tube the clotting of horseshoe crab blood cells when they touch bacterial components.

The reaction runs as a chain of enzymes. Endotoxin first activates factor C at the entrance, factor C activates the next factor, and so on until a protein finally clots. The reagent made from blood cells is LAL (Limulus amebocyte lysate). It can be read by gel clot, by turbidity or by color change. rFC makes only factor C, the entrance to the chain, by recombinant technology. It reads the amount of endotoxin from the intensity of light released when a fluorogenic substrate, added beforehand, is cleaved. A reagent that makes every step of the chain recombinant is called a recombinant cascade reagent (rCR).

LAL has a second pathway that also responds to beta-glucan, and its sensitivity can shift from lot to lot. rFC has no beta-glucan pathway. rCR reproduces the same chain as LAL, but recombinantly. As examples of products, Lonza's PyroGene and bioMérieux's ENDOZYME II GO are rFC reagents.

rFC avoids beta-glucan false positives because it has no beta-glucan pathway to begin with, but that does not mean it escapes every other kind of interference.

Why switch? Horseshoe crabs, supply and false positives

There are three motives.

The first is the effect on horseshoe crabs. The aims of developing recombinant reagents, as listed in the Japanese Pharmacopoeia (JP) general information chapter, are protecting horseshoe crabs, stabilizing reagent supply, removing lot-to-lot differences and making the test more stable. The ASMFC puts short-term mortality after bleeding at 15% (a range of 4 to 30%). Applied to the capture estimate above, that works out to roughly 80,000 animals dying each year.

The second is supply. A reagent that depends on natural blood cells is exposed to harvest regulation and to the state of the stock. A recombinant reagent can keep being made to the same design, so lot-to-lot differences are expected to be smaller.

The third is avoiding false positives from beta-glucan. LAL also responds to beta-glucan that leaches from materials such as cellulose filter media, and the value can read high even though endotoxin has not increased. rFC has no such pathway, so finding the cause is simpler.

On the regulatory side, the European Pharmacopoeia carries general chapter 2.6.32. The USP approved general chapter <86> in July 2024 and made it official in May 2025. Chapter <86> is a method added to the existing <85>, so a company that uses LAL has no obligation to change. The JP also has a general information chapter (参考情報) on an alternative method that uses recombinant proteins.

There is no obligation to switch and the reasons differ by company, so settle your purpose first, because it decides how much you have to confirm.

What should you compare before you switch?

Compare four things.

The first is whether your own product is measured the same way. The reliability of the test depends less on the reagent specification than on how it fits the components of your formulation. A study of rFC testing on biopharmaceuticals reported that LAL and rFC showed different interference depending on the sample, and that dilution solved some of the cases. With some combinations of buffer, surfactant and chelator in the formulation, interference changes when the reagent changes. Low endotoxin recovery (LER), in which endotoxin becomes hard to measure in a formulation, can also occur with rFC, since the reaction starts at factor C either way (see low endotoxin recovery (LER) (in Japanese)).

The second is the instrument and the procedure. rFC is read by fluorescence, so you need a plate reader and software that support fluorescence. If you are moving from a simple format such as gel clot, you take on both instrument adoption and the checks that come with electronic records (see data integrity (in Japanese) and the plate reader directory).

The third is the approval document and change control. For an approved product, changing the test method changes what the approval document says. It goes through your internal change control and, depending on the market, a procedure with the authority (see change control (in Japanese) and ICH Q12 (in Japanese)). A new product in development can be designed around a recombinant reagent from the start.

The fourth is the case where testing is outsourced. Does the contract lab support recombinant reagents, and has it checked them on your product? If it does not, the timing of your switch has to follow the lab's.

The basis for the decision is not the maker's documents but a result on your own product, measuring the same sample with LAL and with the recombinant reagent side by side. Compare spike recovery, recovery after a hold time, and whether the dilution series falls on a straight line, with both reagents.

Whether to switch is decided by fit with your own product and by whether you can run the change procedure, not by how well the reagent performs.

What goes wrong when labs switch?

Five mistakes come up.

The first is to assume that changing the reagent is enough and to skip confirming the fit for each product. Interference differs by product, and changing the reagent changes the premise.

The second is to switch without a side-by-side comparison with LAL. When the trend of results shifts, you cannot tell whether the product changed or the reagent differs, and trend control falls apart.

The third is to feel safe because the reagent is not animal-derived and to overlook other interference such as LER.

The fourth is for purchasing or the laboratory to move ahead and put QA's change control through afterward. The timing of the reagent purchase contract and of the approval-document procedure drift apart, and the switchover schedule stalls.

The fifth is to rely on a single supplier of the reagent. Confirm at least one alternative reagent and format in case supply stops.

Changing the reagent is a change of test method, so run it as validation and change control, not as a purchase of reagent.

Checks you can run tomorrow

Look at your own laboratory or your contract lab and check the following.

1. Which LAL format (gel clot, turbidimetric or chromogenic) does your current endotoxin test use, and for which products or water?
2. Have you decided why you would switch to a recombinant reagent (care for horseshoe crabs, lot-to-lot differences, beta-glucan)?
3. Do you plan to confirm spike recovery and recovery after a hold time with the recombinant reagent on your own products?
4. Do you hold, for each product, comparison data from measuring the same sample side by side with LAL?
5. Has the fluorescence-capable instrument and software completed qualification?
6. Have you worked out change control and the market-by-market procedures for changing the test method stated in the approval document?
7. Does your contract lab support recombinant reagents?

If you are in purchasing or sales, ask the reagent maker two questions. Do you have experience or documents showing that interference was studied with the components of our formulation? What is your fallback if supply stops? Candidates for reagents and instruments can be compared in the endotoxin test directory.

How to set the limit, the ceiling on dilution and how to catch interference are in endotoxin testing in practice, and the endotoxin limit calculator works out the limit. The nature of endotoxin itself is in what endotoxin is (in Japanese), and how it differs from bioburden in bioburden versus endotoxin (in Japanese).