The Test That's Actually Failing - 3 of 6 Failed Sterility at 99.8%

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View this post on the web at https://derekpruski.substack.com/p/the-test-thats-actually-failing-3

Yesterday I ran a poll asking which test you’d add on top of identification, purity, and net content.
127 votes came in:
Endotoxins — 55 votes (43%)
Sterility — 50 votes (39%)
Heavy Metals — 22 votes (17%)
Endotoxins won. And honestly, that’s a reasonable choice — endotoxins are well understood, they have clear pass/fail thresholds, and they’re directly relevant to safety.
But here’s what the real-world data is actually showing: sterility is the test that’s failing more than anything else across vendors right now.
Not at alarming rates. But it’s happening — and it’s happening on batches where everything else looks perfect.
Peptidology just shared their MOTS-C COA with me (Lot 1670), and it’s a textbook example of exactly this. Let’s walk through it.
Everything else on this batch passed cleanly.
Vanguard Laboratory ran a comprehensive panel on 2/26/2026:
Chromatographic purity (HPLC-UV/VIS): >99.80% across all three vials tested
Assay/net content: 10.36 mg, 10.32 mg, 10.36 mg — labeled at 10 mg, so slight overfill, which is normal and good
Heavy metals (ICP-MS): chromium, arsenic, cadmium, lead, and mercury all non-detect
Endotoxins (LAL): both samples passed at <5.00 EU/mg, well under the USP/FDA threshold
Residual solvents (GC-MS): all 16 solvents tested — benzene, chloroform, dichloromethane, acetone, ethanol, and more — all non-detect
TFA: non-detect, below 0.10%
Solubility: fully soluble in 2 mL water on both samples tested
Container closure integrity (dye ingress): both passed
On paper, this is as clean as a COA gets.
Then you get to sterility.
Vanguard ran USP <71> sterility testing on five samples from this lot. Three failed — growth detected. Two passed — no growth.
Eagle Analytical Services ran an independent ScanRDI sterility test on a separate sample from the same lot. That one passed.
Six total sterility tests across two independent labs. Three fail, three pass. Same lot. Same batch.
This is not a vendor failure. This is sterility testing doing exactly what it’s supposed to do — and showing you how complicated this actually is.
Why can samples from the same lot give different sterility results?
Sterility isn’t a property of the compound itself. MOTS-C doesn’t inherently have bacteria in it. What sterility testing is detecting is contamination — and contamination is not uniform.
Think of it like this: if you dropped a single grain of dirt into a swimming pool, some water samples would test positive and some would test negative depending on where you pulled from. The contamination exists somewhere, but it’s not evenly distributed.
A few mechanisms that explain variable results on the same lot:
1. Environmental contamination during fill/finish The peptide compound itself may be clean. But the process of filling vials and lyophilizing (freeze-drying) happens in a controlled environment — and that environment isn’t always perfect. A brief lapse in cleanroom conditions, a failure of laminar flow, an unsterilized surface — any of these can introduce contamination into some vials but not others.
2. Low-level contamination below detection threshold The contamination load may be very low — a small number of organisms. If only a handful of bacteria are present, the test may detect it in some samples and miss it in others depending on exact volume tested and starting organism count.
3. Two different methods were used here The Vanguard results used USP <71> — a culture-based method that incubates samples for 14 days to see if anything grows. Eagle used ScanRDI, which stains and filters the sample and counts organisms directly under fluorescent light in roughly 24 hours. These two methods can give different results on borderline samples. Neither is wrong — they’re measuring slightly different things with different sensitivities.
4. Sampling variability For a 10 mg vial, the tested portion is a fraction of total content. If contamination is localized to a specific area of the vial, some draws will capture it and others won’t.
What does this mean for researchers?
A few takeaways:
A single sterility pass doesn’t guarantee the entire lot is sterile. This is why testing multiple samples and using independent labs matters. Peptidology sent samples to two different labs — that’s the right approach.
Purity >99% tells you nothing about sterility. These are completely separate things. A peptide can be analytically perfect and still have microbial contamination introduced during manufacturing. The argument “if purity is high, everything else is fine” doesn’t hold.
Sterility failures don’t automatically mean a vendor has bad practices. It means contamination is hard to eliminate entirely and even harder to detect consistently. What matters is whether the vendor is testing at all, how many samples they’re running, and what they do when failures show up.
Endotoxins and sterility are not the same test. Endotoxins (LAL) detect fragments from gram-negative bacterial cell walls — they can be present even if all live organisms are dead or removed. Both tests matter independently, and both passed on this COA.
The bottom line
You voted endotoxins as the most important additional test. That’s not wrong — endotoxins are important and worth testing for.
But if you’re looking at what’s actually coming back with failures on otherwise clean product right now, sterility is making the strongest case for itself.
Peptidology sharing this data publicly — including the failures — is exactly the kind of transparency the research community should be pushing vendors for. The uncomfortable results matter more than the clean ones.
All compounds referenced are for research purposes only, not for human consumption.

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