PharmD_Rodriguez said:

The pattern that distinguishes a bad batch from an exit is behaviour rather than product.

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.

Dr.Martinez said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

Shipping damage protocol for vendor vetting orders: if your package arrives with visible damage, temperature excursion, or broken vials:

• Photograph EVERYTHING before discarding packaging
• Note the temperature of the cold pack (liquid = possibly too warm)
• Contact the vendor within 24 hours with photos
• Request a replacement shipment or refund
• Do NOT use potentially compromised medication

Reputable vendors will replace damaged shipments without argument. If they push back, that's a red flag about the vendor.

LabKate said:

I disagree that testing history is decisive.

Coming at LabKate’s question from a different direction. Denials are usually procedural rather than clinical, and the order that works reflects that. Get the denial reason in writing, because it names the criterion you failed. Then supply the documentation that criterion asks for — usually documented BMI with a comorbidity, or a failed prior therapy. Then appeal, and ask for a peer-to-peer review, because a prescriber talking to a reviewing clinician resolves a large fraction of denials that written appeals do not. Manufacturer copay assistance is separate and applies mainly to commercial insurance, and patient assistance programmes are means-tested rather than a discount.

A narrower follow-up, since the general answer is now clear:

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.