Of all the lines on a bacteriostatic water certificate of analysis, the endotoxin result is the one most often misunderstood, because a product can be perfectly sterile and still fail it. This guide explains what endotoxins are, why they are tested separately from sterility, how the test works and what the limit means.
What endotoxins are
Endotoxins are lipopolysaccharides (LPS), structural components of the outer membrane of Gram-negative bacteria such as E. coli and Pseudomonas. They are released when those bacteria die and break apart. Wherever Gram-negative bacteria have grown, in a water system, a storage tank, a length of tubing, endotoxin remains behind after the bacteria themselves are gone. Endotoxin is also called a pyrogen, and endotoxin testing is sometimes called pyrogen testing.
Why sterility does not cover it
Sterilization kills organisms; it does not remove their debris. Endotoxin is extremely heat-stable, surviving autoclaving and boiling, and it is far smaller than a bacterium, so it passes through 0.22 µm sterilizing filters. Water can therefore be sterile and still carry a high endotoxin load if it was contaminated before sterilization. That is why the pharmacopeial water grades treat the two as separate requirements: a sterility test for living organisms, and an endotoxin test for what they leave behind. Removing endotoxin from water requires distillation, reverse osmosis or ultrafiltration in a system operated to prevent biofilm; removing it from glassware requires dry heat at around 250 °C.
The unit: EU/mL
Endotoxin is measured in endotoxin units (EU) per millilitre, a unit of biological activity referenced to an international standard rather than a mass, because different endotoxins have different potencies. The limit for bacteriostatic water is not more than 0.5 EU/mL; the limit for WFI-grade and sterile water is 0.25 EU/mL. A typical result on a well-made lot is well below the limit and is often reported as "< 0.25 EU/mL" or "< 0.05 EU/mL", reflecting the sensitivity of the method used.
How the LAL test works
The Limulus amebocyte lysate (LAL) test uses an extract of blood cells from the horseshoe crab, Limulus polyphemus, which contains a clotting cascade that is triggered by minute amounts of endotoxin. There are three formats:
- Gel-clot: the sample is mixed with lysate and incubated at 37 °C; a firm gel that holds when the tube is inverted indicates endotoxin at or above the lysate's labeled sensitivity. Simple and qualitative, run against a series of dilutions.
- Turbidimetric: the rate at which the mixture becomes turbid is measured photometrically and compared to a standard curve, giving a quantitative result.
- Chromogenic: the lysate cleaves a synthetic substrate to release a colored product; the color development rate is read against a standard curve. Also quantitative.
Every run includes controls: a negative control (endotoxin-free water), a positive product control (the sample spiked with a known amount of endotoxin, to prove the sample itself does not inhibit or enhance the reaction), and a standard series. A recombinant version of the key enzyme (recombinant Factor C) is increasingly used as an animal-free alternative with equivalent results.
What to look for on a COA
- The test named: "Bacterial endotoxins (LAL)" or equivalent, with the method (gel-clot, kinetic turbidimetric or kinetic chromogenic).
- The specification: NMT 0.5 EU/mL for bacteriostatic water.
- A numeric or "less than" result, not simply "pass".
- A lot number matching the vials.
The absence of an endotoxin line is a red flag: it means either the lot was not tested or the supplier is not reporting it. The full COA walkthrough is in how to read a bacteriostatic water COA; the companion sterility test is explained in sterility testing explained.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.