Microbiological testing

Microbiological testing covers two distinct questions. Bioburden testing enumerates the viable organisms present in a sample. Sterility testing asks whether any viable organisms can be recovered under specified culture conditions. They answer different questions and are not substitutes for one another.

The limits of sterility testing

Sterility testing examines a sample, not the whole batch, and is therefore a statistical exercise. Because contamination is typically sparse and unevenly distributed, a passing sterility test on a modest sample provides only limited assurance about the batch as a whole. This is why assurance of sterility in regulated manufacture rests primarily on validated process control, with testing as a confirmation rather than the basis of the claim.

Culture-based methods also detect only organisms that grow under the conditions used. Organisms with different requirements, or those present in a stressed and non-culturable state, may not be recovered.