Dr.RaviCardio 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.

DoseLogDan said:

I have a certificate reporting 99.2% purity and 6.1mg content on a vial sold as 5mg, and I cannot work out whether those two numbers are consistent or…

My vendor review relevant to certificates of analysis: ordered 5 times from my current compounder. Consistency has been excellent.

Shipping: Same-day local pickup
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 97.2% average across orders (Janoshik verified)
Customer service: Responsive within 6 hours
Price: $146/month for semaglutide 2.4mg equivalent

Dr.PainCLE said:

I am more sceptical of certificates as a category than this thread is.

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 61 submitted tests:

• 78% tested at ≥98% purity
• 15% tested at 95-98% purity
• 7% tested at <95% purity (concerning)

The vendors with consistently high purity results are listed in the Vendor Directory. We update monthly based on new community submissions.

Following on from lisa_labSD — and this may be the naive question:

Why two laboratories report different purity for the same batch, and which number to believe?

Closing the loop on my own question.

Purity and content being different measurements was the piece I was missing. The vial was overfilled and pure, which is a very different problem from the one I thought I had.