Why did two CPC lots of tirzepatide differ on content assay?

The case in front of me: CPC · tirzepatide.

I think I have a problem. I am not yet sure whether it is a real problem or a measurement artefact.

I want to know whether this is recoverable or whether the honest answer is to write it off.

Should I be treating this as a failure or as noise?

Two vials tested from a lot of ten is very different from two vials tested from a lot of ten thousand, and most certificates do not state the lot size.

A statement that "lot 20260412 complies with specifications" is meaningless without stating which vials from the lot were tested and how many there were.

What each test answers

Put another way, stratified sampling — testing one vial from the top, one from the middle, and one from the bottom of a shipment — is cheap insurance against segregation.

Published data on lot homogeneity from manufacturers who sample multiple vials consistently find variation below the published specifications, suggesting the sampling plans work.

If testing multiple vials, state how many you tested and why you chose those vials.

Mechanically, batch testing establishes what can be claimed about the lot as a whole, and the sample size determines how much you can actually claim.

If the lot was manufactured in multiple batches, testing vials from each batch separately establishes whether batch-to-batch variation is acceptable.

Stated carefully, the statistical foundation here is well-established, which is why sampling plans from decades ago are still valid.

One qualification: testing more vials gives better confidence, but at some point the cost outweighs the benefit.

The practical summary: a lot number without a sampling statement is a lot number without meaning.

The part that matters: thermal history during shipping is different for every vial, so a lot that experienced thermal abuse may have internal variation even if it was originally homogeneous.

Under AQL sampling plans, testing two vials from a fifty-vial lot gives you an operating characteristic curve that tells you what risks you are accepting.

Concretely, testing a vial that has been open in the lab for three months is testing aged material, not the fresh lot, and the result should be explicitly noted as a retest.

Lyophilised peptide homogeneity studies show that vial-to-vial variation is usually small but occasionally large, depending on the distribution in the freeze-dryer.

I would treat a "complies with" statement without sampling details as a claim rather than as evidence.

Assume segregation is possible, and design your sampling to catch it if it exists.

edited 28 Apr 2025 by forty_units — added the placebo-arm figures