Endotoxin and sterility testing

HPLC purity and mass-spectrometry identity answer how pure and which molecule. Two further tests sometimes appear on a Certificate of Analysis and answer a different question — whether the material carries biological contaminants. These are endotoxin and sterility testing.

What endotoxin is

Endotoxins are components of the outer membrane of certain bacteria. Even when the bacteria themselves are gone, these fragments can remain, and they can interfere with sensitive biological assays. Endotoxin testing measures how much is present, usually reported as endotoxin units per milligram (EU/mg), with a low figure (for example, <0.05 EU/mg) indicating a low level. For cell-culture and other sensitive in-vitro work, a low endotoxin result is one less confounding variable.

What sterility testing checks

Sterility testing checks for the presence of viable microorganisms — a "no growth" result indicates none were detected under the test conditions. It's a distinct question from purity: a chemically pure sample could still, in principle, carry microbial contamination, which is why it's tested separately.

Where these sit in the bigger picture

Think of a complete quality picture as four separate questions, each with its own test:

• How pure? → HPLC
• Which molecule? → mass spectrometry
• Any endotoxin? → endotoxin assay
• Any microbial growth? → sterility test

No single test covers all four. A strong certificate reports the ones relevant to the material, all tied to the same lot code so the results describe the exact vial you'll receive. More on reading a certificate here.

The takeaway

Endotoxin and sterility results tell you about biological cleanliness, complementing the chemical picture from purity and identity testing. As with every other result, they're only meaningful when tied to a specific lot. You can look up a DNR batch to see the certificate on file, or browse the catalogue. For laboratory and research use only.