Sterility Testing on a Peptide COA: PCR Screens vs USP 〈71〉

"Sterility" on a research-peptide certificate can mean two quite different tests. One is the compendial culture test in USP general chapter <71>, which waits to see whether anything grows. The other is a molecular screen, usually PCR, which looks for microbial DNA. They are worth telling apart, because each one establishes something the other cannot.

The compendial test: USP <71>

USP <71> is a growth test. The sample is placed into culture media, the media are incubated, and the analyst watches for organisms to multiply (USP <71>).

Two techniques

• Membrane filtration. The sample is passed through a membrane filter that retains microorganisms, the membrane is rinsed (more than once if the product has antimicrobial properties), and then the membrane goes into the culture media. The chapter makes this the technique of choice whenever the material can be filtered.
• Direct inoculation. A measured quantity of the sample goes straight into the culture media.

Two media, fourteen days

The chapter pairs fluid thioglycollate medium, intended mainly for anaerobic bacteria but also able to detect aerobic ones, with soybean–casein digest medium, which supports fungi and aerobic bacteria. Each is incubated at its own temperature range for not less than 14 days. The analyst examines the cultures during and at the end of incubation for macroscopic evidence of growth. The chapter also sets minimum quantities of material and minimum numbers of containers, and it requires negative controls.

That fortnight is the method's main practical cost: a clinical-microbiology review describes the turnaround time for compendial culture results as long (Gebo & Lau).

What a pass means, in USP's own words

The chapter is unusually direct about its own limits: a satisfactory result only indicates that no contaminating microorganism has been found in the sample examined under the conditions of the test (USP <71>). It makes a claim about the portion tested, under those media and that incubation. It is not a claim about every container in a lot.

Molecular screens: PCR

PCR (polymerase chain reaction) amplifies specific DNA sequences until there is enough to detect. Applied to contamination screening, it looks for genetic material from microorganisms rather than waiting for them to grow, so results don't depend on a two-week incubation.

What PCR can detect

PCR detects the sequences its assay was designed to target. That has two consequences for how you read a result.

• It doesn't establish viability. A classic study showed that PCR will detect nonviable cells as long as intact target DNA is still present, and noted the risk of positive results that don't reflect living organisms (Josephson et al.). A later clinical-microbiology review describes PCR-positive, culture-negative cases arising from nonviable nucleic acid (Gebo & Lau).
• Its coverage is set by its design. Which organisms the assay targets (bacteria only, or fungi as well), its limit of detection, and how much sample it tested are all properties of that specific assay. These details are often not printed on a certificate.

Rapid methods have to earn equivalence

For regulated products, an alternative rapid microbiological method has to be shown equivalent to or better than the compendial method before it can take its place. Even for commercial molecular assays, the same review notes that in-house validation and method-suitability testing for each product matrix are still required (Gebo & Lau). A PCR screen on a research-material certificate is a useful contamination screen in its own right. Unless the laboratory states that it was validated as an alternative to USP <71>, read it as a screen, not as a compendial sterility result.

"No Growth" vs "Not Detected"

The wording tells you which kind of test produced the result.

• "No Growth" is culture language. It means the media stayed free of visible growth for the incubation period.
• "Not Detected" (or "Negative") is analytical and molecular language. It means the target was not found above the assay's detection limit in the portion tested.

Neither phrase claims more than that. Neither one describes vials that weren't tested, or what happens to material after the container is opened. When a certificate's wording and its stated method don't match, ask the laboratory which test was actually run and what it covered.

Worked example: our certificates

Every lot in our published certificate data carries the same line: Sterility (PCR), specification No Growth, result No Growth, status PASS. Lot BC10-2026-001 of BPC-157 and lot TR10-2026-001 of tirzepatide read the same as the rest.

Here is an honest reading of that line. The analyte name gives the method as PCR, but "No Growth" is the vocabulary of culture testing. That is how ILS Laboratories reports this line, and we reproduce it as printed rather than rewording it.

What the line establishes is that the laboratory's screen of the sample it received did not find contamination by the method it used. What the line does not state is a USP <71> fourteen-day culture result, the organisms the screen targets, or its detection limit. If any of those matter for your work, ask the issuing laboratory. Then confirm the lot's record directly with them using the access code on the certificate (how).

Reading the sterility line on any certificate

1. Which test was run: compendial culture (USP <71>), or a molecular screen?
2. Does the wording match the method: "No Growth" for culture, "Not Detected" for molecular?
3. For a molecular screen, which organisms are targeted, and what is the detection limit?
4. Keep in mind that whatever the result, it describes the sample examined, not every vial in the lot.

For laboratory research use only — not for human or veterinary use.