This article in 30 seconds
- “Safety tested” means different things depending on which layer it refers to. Of the three layers — raw material (the donor), process (the manufacturing environment), and final product — sterility testing of the final product alone is enough to justify the label.
- The three tests cover each other’s blind spots. Sterility testing looks for organisms that live and grow, mycoplasma testing for the bacteria that slip through that net, and endotoxin testing for what dead bacteria leave behind.
- Confirm three things: every lot, down to the donor, and an actual report. Request a Certificate of Analysis (COA) bearing the lot number and you can verify concretely what stands behind the claim.
The big picture — testing splits into three layers
Safety testing of conditioned medium and secretome (the totality of components cells secrete during culture) splits into three layers: raw material, process, and final product. Different layers guard against different risks.
Think of vegetables: the relationship is soil testing in the field (raw material), hygiene control at the plant (process), and inspection before shipping (final product). Because “inspected” can refer to the pre-shipment check alone, the words by themselves do not tell the layers apart.
The key point
In practice, this distinction matters more than anything else. A supplier can run sterility testing on the final product alone and still state “safety tested.” That is not a false claim — but it says nothing about infectious disease risk originating from the donor. That is exactly why you need to confirm which layer was examined, and to what extent.
1. Raw material — donor screening can only stop risk at the entrance
The conclusion is simple: risk originating from the donor who provided the cells cannot be found by examining the finished product. Viruses are not detected by ordinary sterility testing, and there is no guarantee the manufacturing process removes them.
In this layer, everything therefore comes down to screening cells before they are accepted — keeping risk from entering at all. We perform the following screening in line with the manufacturing provisions of Japan’s Standards for Biological Raw Materials (生物由来原材料基準).
| Target | Screening items |
|---|---|
| Viruses | Human immunodeficiency virus (types 1 and 2) / hepatitis B virus / hepatitis C virus / human T-lymphotropic virus (types 1 and 2) |
| Bacteria | Syphilis |
| Prions | Human transmissible spongiform encephalopathy |
Screening for human transmissible spongiform encephalopathy includes exclusion by medical interview. The fact that our process is designed without bovine serum also matters here, as it avoids prion risk of animal origin.
2. Process — not a test, but a layer that lowers the odds of contamination
The essential point about this layer is that it is a matter of facilities and operation, not a test. Rather than catching contamination after the fact, it lowers the very probability that contamination occurs.
We manufacture in Class 100 biosafety cabinets placed inside a Class 10,000 clean booth, operated in line with the environmental standards recommended for the manufacture of regenerative medicine products. Class 100 means no more than 100 particles of 0.5 μm or larger per cubic foot of air.
One caution: cleanliness class is an equipment grade — separate from any test result or certification. This layer is a mechanism for lowering probability; the state of the product itself is verified in the next layer, the final product.
3. Final product — three tests that each look at something different
This is the part of the article we most want you to take away. The three tests are not stacked up “just to be safe” — the relationship is that each looks at something different, which is why all of them are needed.
Sterility testing — sees what lives and grows
The sample is incubated in growth media to see whether microorganisms multiply. Think of it as sowing seeds and checking whether they sprout. It detects bacterial and fungal contamination.
What it does not detect: whatever does not sprout — that is, microorganisms that do not grow under those culture conditions. Mycoplasma (very small bacteria that lack a cell wall) is the classic example. And because dead organisms do not multiply, they are not detected either.
Mycoplasma testing — catches the bacteria that slip through the net
Mycoplasma are bacteria without a cell wall. They are extremely small and are not detected under the culture conditions of ordinary sterility testing. Like fish smaller than the mesh slipping through a net, mycoplasma cannot be ruled out by a pass on sterility testing — a separate test is required.
In cell culture, the particular problem is that contamination tends to go unnoticed while the culture continues. That is why confirmation outside the sterility test is indispensable.
Endotoxin testing — sees what dead bacteria leave behind
Endotoxin (a fever-inducing substance of bacterial origin) is a component of the cell wall of Gram-negative bacteria (a class of bacteria with an outer membrane). Like ash remaining after a bonfire has gone out, it remains after the bacteria die. In other words, a product can pass sterility testing and still contain endotoxin.
Our stem cell secretome tests at below 0.1 EU/mL.
The key point
The three tests cover each other’s blind spots. Sterility testing sees living organisms; mycoplasma testing sees the bacteria sterility testing cannot; endotoxin testing sees what dead bacteria leave behind. Any one of them alone says nothing about what the other two are looking at.
How to read “below” in an endotoxin figure
To state the conclusion first: when comparing statements such as “below 0.1 EU/mL,” where the number comes from matters more than the number itself. Two things are worth checking.
First, is the figure a detection limit or a specification? “Below” can simply mean nothing was detected by that particular assay. Just as a scale with coarser graduations can call a wider range “too light to weigh,” a less sensitive assay lets even larger numbers be written as “below.”
Second, what the value is measured per. EU/mL is a per-volume figure. Just as the same amount of salt gives a different concentration in a glass of water than in a bucket, for products dissolved before use, such as freeze-dried preparations, the meaning depends on the volume in which the product is reconstituted when the value applies.
Three questions for your supplier — verified on the test report (COA)
Everything above condenses into three questions. We issue a lot-specific Certificate of Analysis (COA) for every product and enclose it in the shipment. It reports the results of sterility testing, endotoxin, mycoplasma, and protein concentration.
When comparing suppliers, we recommend asking in the following order.
-
Is every lot tested, or only representative lots?
Unless each lot is measured, nothing can be said about the product actually in your hands.
-
Does testing extend to donor screening?
Final-product testing alone assures nothing about risk at the raw-material layer.
-
Will they actually issue a report bearing the lot number?
Request not a list of test items but the report itself. Whether it can be produced tells you how the system really operates.
In addition, we include a material safety data sheet (MSDS) and a patch test among the items we perform.
Beyond the test items themselves, what to confirm with a supplier — and how — is covered in 6 Questions to Ask Your Supplier; the terminology and an overview of the manufacturing process are covered in What Is a Secretome.
Frequently Asked Questions
If a product passes sterility testing, can I assume it contains no bacteria?
Sterility testing detects microorganisms that grow when the sample is incubated in growth media. Microorganisms that do not grow under those culture conditions are not detected. Mycoplasma is the classic example: it is not found by ordinary sterility testing, which is why a separate mycoplasma test is performed. Likewise, Gram-negative bacteria (a class of bacteria with an outer membrane) that are already dead are not detected by sterility testing, yet endotoxin, a component of their cell walls, remains. The three tests each look at different things, so all of them are needed.
A product just says “safety testing performed.” What should I check?
Check which layer the testing covers. Safety testing splits into three layers: screening of the donor as the raw material, control of the manufacturing process, and testing of the final product. Sterility testing of the final product alone and testing that extends to donor infectious disease screening cover very different ground. Also confirm whether testing is performed on every lot or only on representative lots.
Is a Certificate of Analysis (COA) issued every time?
We issue a lot-specific COA for every product and enclose it in the shipment. It reports the results of sterility testing, endotoxin, mycoplasma, and protein concentration.
This article describes quality control and testing items. Our stem cell secretome is supplied for research use only and is not intended to demonstrate efficacy against any specific disease.
See our safety and quality system Request a Certificate of Analysis