1,386 independent third-party lab assays are published here, covering 879 compounds across 207 vendors. Each one is a certificate of analysis issued by an analytical lab against a specific submitted lot.
What an assay establishes is narrow and worth stating exactly: the identity and purity measured in the sample that was submitted, on the date it was submitted. It says nothing about the next batch, nothing about sterility, and nothing about whether the vial you receive came from the tested lot.
Purity is not identity. A sample can assay at 99% purity and still be the wrong compound entirely, because purity measures how much of the sample is a single substance, not which substance it is. Mass spectrometry on the certificate is what addresses identity; an HPLC purity figure alone does not.
Vendor-submitted testing carries an obvious selection problem — a vendor chooses which lot to send. That is not a reason to ignore the data, but it is a reason to weight a long run of assays over a single flattering one, and to treat a vendor with no testing as untested rather than as failed.
Where a vendor's advertised purity and its median measured purity disagree, both figures are published side by side on the vendor page with the assay count they are drawn from.
Related: Independent lab tests, Vendor directory, How we rate vendors.