Dr.ObesityMed said:

The useful checklist is about verifiability rather than presentation.

I disagree that testing history is decisive. It tells you what a supplier did when they were being watched. Continuity of behaviour under stress — a late shipment, a failed test, a complaint — is more predictive than any run of good results.

Adding the numbers, since they settle part of this. The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as well as purity. A report failing any of those is weak evidence regardless of the percentage on it.

Worth separating that from certificates of analysis, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

JessicaH_TX said:

I disagree that testing history is decisive.

There is a second half to this that has not been said yet. The provenance questions are more useful than the numbers. Does the report name the batch on your vial. Is the date consistent with when that batch existed. Is it addressed to the vendor or to the person who paid for the test. Was the sample taken from general stock or supplied by the vendor. A perfect certificate for a different batch is worth nothing, and vendor-supplied samples are the most common way a genuine report ends up misleading.

Worth separating that from certificates of analysis, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

I would rather be corrected than agreed with, if it comes to it.

One thing that is still open after josh_phd_bmore’s answer:

How people are distinguishing a supplier having a bad batch from a supplier on the way out?

OP back with an update, since a thread like this is useless without one.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.