A sample can pass HPLC purity analysis and mass spectrometry identity confirmation — and still carry contamination that neither method can detect. Endotoxin and sterility testing exist to catch what chemical analysis misses: microbial contaminants.
What endotoxins are
Endotoxins are fragments of the outer membrane of certain bacteria (gram-negative bacteria, technically). They are not living organisms — they are toxic remnants that persist even after the bacteria themselves are dead. This is what makes them tricky: sterilizing a sample does not remove endotoxins that were already present.
Endotoxin levels are measured in Endotoxin Units (EU), typically with a method called the LAL test (Limulus Amebocyte Lysate), which uses horseshoe crab blood cells that clot in the presence of endotoxins. Results are reported as EU per milligram or EU per milliliter, against established safety thresholds.
What sterility testing covers
Sterility testing answers a simpler question: are there living microorganisms — bacteria, fungi — in the sample? The sample is placed in growth media and incubated; any microbial growth indicates contamination.
Related tests you may see on reports include:
- Total aerobic microbial count — how many living microbes are present
- Yeast and mold count — fungal contamination specifically
- Pathogen screening — tests for specific harmful organisms like E. coli or Salmonella
Why microbial contaminants distort laboratory analysis
Even in purely analytical settings, microbial contamination is a serious problem:
- Bacteria metabolize compounds — a contaminated sample chemically changes over time, so repeat measurements drift
- Endotoxins trigger biological responses — any experiment involving cell cultures or biological systems will react to the contamination, not just the target compound
- Results become irreproducible — two labs testing the same contaminated batch can get different answers
In other words, microbial quality is not just a safety concern — it is a data integrity concern. Analysis performed on contaminated material produces unreliable results, whatever the HPLC purity says.
What to look for on a report
- An endotoxin result in EU/mg or EU/mL, with the method named (usually LAL)
- A sterility or microbial count result, not just chemical purity
- Testing performed on the same batch as the purity and identity analysis
- A named, independent laboratory with documented methods
Complete testing documentation — purity, identity, and microbial quality — is the standard we highlight in our guide to why third-party lab reports matter. You can review how listed vendors document their testing in the vendor directory.