A toxic component of the outer membrane of certain bacteria. It survives sterilization, so a sterile product can still carry endotoxin. This is why sterility and endotoxin are two separate tests.

The absence of viable microorganisms. A sterility test cannot detect the presence of sterility; it can only fail to detect growth, which is why results are reported as no growth detected rather than as a pass.

The quantity of microorganisms present, as distinct from a simple sterile-or-not determination.

Elemental impurities such as lead, arsenic, cadmium, and mercury, which can be introduced during synthesis or from equipment.

Solvents left over from the manufacturing process that were not fully removed.

A substance present in a product that should not be there, whether added deliberately or introduced through contamination or poor process control.

Material from one production run appearing in another, usually through shared equipment that was not adequately cleaned.

A vial can be 99% pure and still carry endotoxin, heavy metals, or sterility problems, because purity and contamination measure completely different things.