Dr.PathRoch said:

Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…

That is correct as far as it goes, and here is where it stops going. One addition: an unstated method makes a report close to uninterpretable. "99% pure" without the technique, the standard and the conditions is a claim, not a measurement.

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.

Dr.GutHealth said:

Agreed, with a caveat about community reputation: it is a lagging indicator.

Saving this. It is the first explanation that did not require me to already understand it. Taking it to my next appointment.

Dr.PathRoch said:

Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…

I am more sceptical of certificates as a category than this thread is. The document tells you about a sample somebody chose to send, at a moment in time, and nothing about the vial in your hand. Independent purchase-and-test is a different quality of evidence and everything else is inference.

maria_elpaso said:

Eight months with one supplier, then a change in how they communicate that I noticed before I noticed anything about the product.

Vendor reputation tracking for vendor vetting: I maintain a personal spreadsheet with every order I've placed — date, vendor, batch number, Janoshik result, personal efficacy assessment (1-5 scale), and any issues.

After 19 orders across 4 vendors: average purity 98.3%, zero sterility concerns, 2 shipping issues (all resolved). This data gives me confidence in my current source and documentation if issues arise.

I encourage everyone to track their orders systematically. Community data aggregation makes us all safer.

Moderator note: good thread. Keeping it here rather than moving it, because the question is general enough to be useful.