LAL endotoxin

Which endotoxin methods do the pharmacopoeias recognise?

The bacterial endotoxins test is harmonised across the European, Japanese and US pharmacopoeias. ICH Q4B Annex 14 records that Ph. Eur. 2.6.14, JP 4.01 and USP General Chapter <85> can be used as interchangeable in the ICH regions (ICH Q4B Annex 14). The harmonised text, published by the European Medicines Agency as part of the Q4B process, describes a lysate made from the blood cells of horseshoe crabs (Limulus polyphemus or Tachypleus tridentatus) and two families of technique: gel clot, based on gel formation, and photometric, based on optical changes, the latter split into turbidimetric and chromogenic (harmonised Bacterial Endotoxins Test text, EMA).

What an EU is and how to read an endotoxin line are covered in the lesson on endotoxin and sterility. This page compares the techniques.

How do the four main variants compare?

The harmonised text also describes endpoint versions of the turbidimetric and chromogenic methods, which read the signal at one fixed time instead of tracking it.

How does a gel clot test give a number?

Each lysate lot carries a labelled sensitivity, written λ (lambda), the lowest endotoxin concentration that will form a gel. In the gel clot limit test, a sample diluted by a known factor either clots or does not. A negative result means the endotoxin concentration is below λ times the dilution factor. The quantitative gel clot assay runs a dilution series and takes the last positive tube as the endpoint. The harmonised text requires the labelled sensitivity to be confirmed: the geometric mean endpoint of a standard series must fall between 0.5λ and 2λ.

Gel clot needs no plate reader, but its resolution is tied to the dilution steps, so a result is usually written as "less than" a value.

How do kinetic methods turn time into endotoxin units?

In a kinetic assay, more endotoxin makes the reaction reach its threshold sooner. The laboratory runs endotoxin standards at several concentrations, plots the log of onset time against the log of concentration, and reads samples off that line. A proficiency-testing study comparing LAL with rFC describes this for a kinetic chromogenic LAL test, where onset times fall as endotoxin concentration rises and both axes are logarithmic (Piehler and colleagues, Microorganisms, 2020).

Dilution sets the floor. The harmonised text defines a maximum valid dilution: the furthest a sample can be diluted while the assay can still detect the endotoxin limit. Diluting further could hide endotoxin.

How is interference from the sample checked?

Every quantitative run includes a positive product control: the sample spiked with a known amount of endotoxin. The harmonised text treats the sample as free of interfering factors when recovery of that spike is within 50% to 200%. Outside that range, the laboratory dilutes further (within the maximum valid dilution) or removes the interference by a validated step such as filtration or neutralisation. Piehler and colleagues report that LAL and two rFC methods all gave valid results across their proficiency samples by this 50% to 200% criterion.

What is recombinant factor C, and is it accepted?

LAL works because endotoxin activates factor C, the first enzyme of the horseshoe crab clotting cascade, which then activates factor B and the clotting enzyme in turn (Iwanaga, Proceedings of the Japan Academy, 2007). The same lysate contains factor G, which responds to (1,3)-beta-D-glucan from fungi. An rFC reagent contains only a recombinant form of factor C, so the glucan pathway is absent. Piehler and colleagues describe this simplification as a first level of specificity for endotoxin, and note that fluorescence reading gives a wide dynamic range of about four orders of magnitude without needing onset times.

A 2018 review in PLOS Biology, which tabulated ten studies, found commercially available rFC tests detected endotoxin with results equivalent to or better than LAL, noted that rFC had been commercially available since 2003, and reported that the US FDA issued guidance in 2012 acknowledging rFC as an acceptable alternative to LAL (Maloney and colleagues, PLOS Biology, 2018). The current FDA questions and answers guidance on pyrogen and endotoxins testing says firms may use alternative methods if they are validated and shown to achieve equivalent or better results, and that in a dispute the compendial gel clot method decides unless the monograph says otherwise (US FDA, Pyrogen and Endotoxins Testing Q&A, Edition 2, 2026).

Maloney and colleagues add that European regulators followed the FDA in accepting rFC, while describing industry uptake as slow. Because the standing of recombinant reagents differs between pharmacopoeias, a record that uses rFC should name the chapter or validation it relies on.

How does Tested.ph offer endotoxin testing?

Bacterial endotoxin by LAL assay is offered as an add-on to the standard package and as an endotoxin-only package. An endotoxin-only record does not establish identity, purity or amount. The partner laboratory's exact technique will be stated on each record when testing opens. See pricing for what each package covers.