USP 61 is not a sterility test

Enumeration is cheaper, faster and does not require an isolator. A laboratory without barrier capability can run TAMC and TYMC on ordinary bench equipment, and if the counts come back at zero it is tempting to render that as a pass.

A pass framing also removes an inconvenient number. Counts are informative in a way a binary result is not: ten colony forming units per gram and nine hundred both sit under many specifications while saying entirely different things about the process that produced them. Collapsing that into pass discards the information the test exists to generate.

The consequence lands on injectables. A buyer told an injectable preparation passed sterility, when what was performed was enumeration, believes the product carries an assurance it has never been given.

Neither result means anything until the method has been shown to work on that specific product. Many materials are antimicrobial, whether through a preservative, a solvent residue or the active itself, and a suppressed plate looks exactly like a clean one.

Suitability is demonstrated by inoculating the product with a low-level challenge, typically no more than 100 colony forming units, and showing recovery within a factor of two. Where recovery fails, the method is modified by dilution, filtration or neutralisation until it succeeds.

The more antimicrobial the product, the more likely an unsupported clean result is an artefact of chemistry rather than evidence of quality.

Endotoxin is lipopolysaccharide from the cell wall of Gram-negative bacteria. It is a molecule, not an organism, and it survives autoclaving and passes through a sterilising filter.

A product can therefore be sterile and pyrogenic at the same time. Endotoxin requires its own determination under USP <85> or, using recombinant Factor C, USP <86>. Three separate questions, three separate tests, and no two of them substitute for the third.