Endotoxin and microbial testing explained

Three separate questions get compressed into the word “clean”. They use different instruments, produce different units, and only one of the three is offered by this laboratory.

By Canada Peptide Testing. Published September 1, 2026, updated September 13, 2026. 8 minute read.

When someone asks whether a preparation is clean, they are usually asking one of three things without knowing they have to choose. Are there living organisms in it? Is there bacterial residue in it, whether or not anything is still alive? Or is it, in the formal sense, sterile? These are not degrees of the same measurement. They are different measurements with different failure modes.

Endotoxin: a residue, not an organism

Bacterial endotoxin is lipopolysaccharide from the outer membrane of Gram-negative bacteria. The reason it is measured separately from the bacteria themselves is that it is remarkably durable: it survives autoclaving, it survives filtration that removes cells, and it persists in material long after every organism that produced it is dead.

That durability is the whole point. A preparation can be free of viable organisms and still carry endotoxin from a contamination event that happened upstream and was subsequently killed. Testing for organisms would find nothing; testing for endotoxin finds the record of what was there.

How the measurement works

Endotoxin triggers an enzymatic cascade in a lysate reagent. In the kinetic format, the assay measures how long the reaction takes to reach a defined optical threshold and interpolates that time against a standard curve built from reference endotoxin at known concentrations. Faster onset means more endotoxin. The result is expressed in endotoxin units, usually per millilitre.

The control that makes it interpretable

Some matrices interfere with the cascade, in either direction. A sample that inhibits the reaction produces a clean-looking result that is an artefact. That is why a positive product control is run alongside: a known amount of endotoxin spiked into the sample matrix, whose recovery has to land inside an acceptance window. Its recovery is reported on the certificate, because a clean endotoxin result without it is an unfinished statement.

The related constraint is maximum valid dilution. Diluting a sample reduces interference, and it also reduces the assay’s ability to detect endotoxin at the limit being tested against. Past a calculable point, a pass stops meaning anything, so dilution is bounded by that calculation rather than by convenience.

Microbial enumeration: not offered here

This laboratory does not offer microbial quality testing. It is described here only because it is the measurement people most often mean when they ask about endotoxin, and knowing the difference is what stops an endotoxin certificate from being read as something it is not.

Microbial enumeration takes the opposite approach to endotoxin. A prepared portion is plated onto growth media and incubated under defined conditions, and colonies are counted, usually as TAMC, total aerobic microbial count, and TYMC, total yeast and mould count. The result is in colony-forming units, and the phrase is deliberately careful: one colony represents one viable organism or clump capable of growing under those conditions. It is a count of what grew, not a census of what was there, and only organisms suited to the chosen media, temperature, and incubation period are counted at all.

If a microbial count is what your specification calls for, it is a separate laboratory’s work. Nothing on an endotoxin certificate issued here substitutes for it.

Sterility: also not on offer

Sterility means the absence of viable organisms, and it is established by a specific compendial sterility test under defined conditions with its own statistical basis. It is a different test, and Canada Peptide Testing does not offer it.

The reason this matters is that endotoxin and microbial results are frequently read as adding up to sterility. They do not. A low endotoxin figure says a bacterial residue is below a stated limit. A zero colony count says nothing grew under those conditions. Even together they would be two pieces of favourable evidence and still not the conclusion, which is why the endotoxin certificate states the distinction explicitly rather than leaving it to be inferred.

What a pass requires

Neither result is a pass or a failure on its own. A pass requires an acceptance criterion, and an acceptance criterion has to come from somewhere: a compendial monograph, a customer specification, a regulatory requirement for the intended use. Where a client supplies one, the certificate states the comparison against it and names its source. Where none is supplied, the measured value is reported and no threshold is implied.

A laboratory that stamps “pass” on a contaminant result with no named specification has decided on the client’s behalf what limit matters, which is not a decision it is in a position to make.

When these are worth ordering

Contaminant screening is not a thoroughness upgrade on a peptide panel; it answers a genuinely different question. It is worth ordering when the material’s handling or storage history makes contamination plausible, when a diluent is involved, when a supplier is new and you are establishing a baseline, or when a customer specification requires the figures. It is not worth ordering because a purity result was good and you want to be thorough; those two facts are unrelated.

Common questions