Endotoxin Testing and Why It Matters in the Lab

A compound can be very pure and still carry bacterial endotoxins. That is why many researchers look for endotoxin testing in the lab alongside the purity report. So, here is a plain guide to what the test does and why it matters.

What endotoxins are

Before any endotoxin testing in the lab, it helps to know what endotoxins are. They are fragments of the outer wall of certain bacteria. They are very stable, and they survive many common cleaning methods. Also, they can upset cell-based work even in tiny amounts. For this reason, labs that run cell studies pay close attention to them. Because endotoxins come from bacteria, they can enter a batch at many stages. For example, water, tools or packing steps can all add them. So, a test on the finished batch gives the most useful answer.

How endotoxin testing works in the lab

The most common method is the LAL assay. It uses a reagent that reacts when endotoxins are present. Then, the lab reports the result in endotoxin units, or EU, usually per milligram or per vial. Lower is better, and the report often shows the result against a stated limit.

Why endotoxin testing is a separate lab test

First, HPLC measures chemical purity. However, it cannot see endotoxins, because they appear in amounts far below what HPLC can detect. As a result, a sample can show 99%+ purity on HPLC and still fail an endotoxin limit. In short, the two tests answer two different questions.

What to look for on the report

• Source: the report should come from an independent lab.
• Sample ID: it should match your batch number.
• Result: look for clear units and the limit the lab used.

When your lab relies on endotoxin testing, check our reports first. Where an endotoxin report exists for a batch, our Lab Results page lists it next to the COA. Meanwhile, batches still in testing show the COA only until the results arrive. You can also read our guide to HPLC purity for the other half of the picture. For lab research use only. Not for human or animal use.