Endotoxin, sterility and purity: three different tests

What is an endotoxin test?

An endotoxin test is a laboratory test that detects endotoxin, a large molecule called lipopolysaccharide that sits in the outer membrane of gram-negative bacteria, a broad group of bacteria sorted by how they react to a staining dye (FDA Inspection Technical Guide No. 40, 1985). The usual version is the bacterial endotoxins test, also called the LAL test after Limulus amebocyte lysate, a test reagent (a chemical that reacts to endotoxin). FDA accepts the LAL test in place of the older rabbit test, and its Edition 2 question-and-answer guidance (March 2026) describes three ways to run the test: gel clot, photometric (light-based) and kinetic (rate-based). Endotoxin testing looks for bacterial residue, not for living bacteria.

Endotoxin is a trace that outlasts the microbe. The FDA guide says endotoxin is released after a bacterial cell dies and breaks open, that sterilizing filters and microbiological filters do not remove it, and that sterile devices have occasionally been found carrying it (FDA, 1985). A test that finds no living microbes therefore cannot stand in for an endotoxin test. Chemists describe lipopolysaccharide as three parts: a fatty part called lipid A, which is the part often named endotoxin, a core chain of sugars and an outer chain of sugars (Raetz and Whitfield, Annual Review of Biochemistry, 2002).

What is a sterility test?

A sterility test checks whether living microorganisms grow from a sample of a material kept under set laboratory conditions. Sterile material is free of viable organisms, meaning organisms able to live and multiply (FDA, Sterile Drug Products Produced by Aseptic Processing, 2004). FDA names United States Pharmacopeia (USP) Chapter 71, the official book of drug-quality standards, as the principal source for sterility test methods. A sterility result therefore describes the units that were sampled. A pass means no contaminating microorganism was found in those units under the test conditions, and it says nothing about the units that were not tested.

The sample is small, and the same FDA guidance says so. Sterility tests are limited in their ability to detect contamination because of the small sample size typically used. As a worked case, a 10,000-unit lot with 0.1 percent contamination, tested with a 20-unit sample, still has a 98 percent chance of passing (FDA, 2004). A passing sterility test is a sampled result, not a guarantee about every vial.

What does a purity test measure?

A purity test estimates the share of a sample that is the intended molecule, set against the other substances in it. The International Council for Harmonisation (ICH), a body that writes shared drug-testing guidelines, says in its Q6B guideline for protein and polypeptide (long amino-acid chain) products (1999) that absolute purity is difficult to determine and that results depend on the method, so purity is usually estimated by a combination of methods. A purity percentage is therefore a statement about chemical makeup, and its meaning depends on the method that produced it. Purity is not a count of microbes and not a measure of endotoxin.

ICH Q6B covers purity, impurities and contaminants together in one section, and it lists sterility and endotoxin among the separate pharmacopoeia tests, the tests set out in official drug-quality standard books (ICH Q6B, 1999). The guideline defines a contaminant as any adventitiously introduced material, meaning material that arrived by accident rather than by design, including microbial species. Chemical purity and microbial testing are written up as different jobs.

Does a high purity figure mean a peptide is sterile?

A high purity figure does not mean a peptide is sterile or free of endotoxin, because purity, sterility and endotoxin are three separate tests that look for three different things. A purity percentage describes the target molecule's portion of the sample. A sterility test looks for living microorganisms, and an endotoxin test looks for bacterial cell-wall residue (ICH Q6B, 1999; FDA, 1985). Each result needs its own test, so a purity result cannot stand in for the other two, and a clean sterility result cannot stand in for purity either.

A certificate claim is also not proof of itself. An independent re-test of 98 synthetic research peptides found that 43 reached the 95.0% purity their certificates claimed (Verbeke et al., Journal of Pharmaceutical Analysis, 2015). That study measured purity only. What a batch or lot number lets a researcher verify explains why a matching code ties a sheet to a run without vouching for the figures on it.

Which of the three tests do the Form Laboratories lot reports document?

None of the 14 lab reports read for this page documents an endotoxin test or a sterility test. All 14 come from one submission to the testing lab Janoshik, ordered on 6 April 2026, received on 10 April 2026 and analysed on 21 April 2026. Twelve list one test, heavy metals analysis, with arsenic, cadmium and lead in a results table and mercury in a comment box, each marked not detected. Two list an assessment of a peptide vial and print a purity figure: 99.511% for the MOTS-c report (batch FL20260410-MOTS-10) and 99.293% for the tesamorelin report (batch FL20260410-TESA-5).

Each report names Formlaboratories.com as client and manufacturer, one batch code and a signature. Ten of the 14 report files are shown on product pages, the MOTS-c research guide among them, and four are not. A missing test line means the test is not documented on that report. It does not mean the test was never run, and these sheets cannot settle that either way. A purity label on a product listing is a catalogue entry, not a laboratory result.

Evidence limits

What do these lot reports not show?

Form Laboratories supplies research material for laboratory research only. The material is not a medicine and is not for human or animal use, and test paperwork of any kind says nothing about safety. Whether any batch carries an endotoxin or sterility result is a question for the supplier, and the answer belongs on a dated report that names the batch and the test.