A sample from each processed lot is sent to a third-party laboratory for sterility testing, separate from the establishment that recovered and processed the tissue. Testing an independent sample from each lot, rather than relying on a single validation study for a product line, is what allows a specific lot's sterility status to be documented individually.
Quarantine is the practical counterpart to sterility testing: a lot is held and not released or shipped while its sample is being tested. Only after results come back meeting the release criteria set for that product is the lot cleared for distribution. This sequence is what the certificate of analysis for a given lot is reporting on when it lists sterility results.
For a buyer, the useful question is not whether sterility testing happens in general but whether it is performed per lot, by a third-party lab, with the lot held in quarantine until results are in. A supplier able to produce that record for the specific lot number on a shipment is providing something more concrete than a general assurance that its process includes testing.
Per-lot sterility testing and quarantine, documented on a matching certificate of analysis, is one of the more concrete pieces of evidence a clinic can request before adding a product to inventory.