Sterility and Endotoxin Testing in Compounding Pharmacies
A plain-language look at the two separate checks that stand behind a safe injectable.
TL;DR
Sterility and endotoxin testing are two different safety checks, and a good compounding pharmacy runs both. Sterility testing confirms nothing living grows from the product. Endotoxin testing confirms it is free of fever-causing fragments left behind by bacteria. Passing one does not mean passing the other.
What sterility and endotoxin testing actually check
Sterility and endotoxin testing answer two different questions about an injectable. Sterility testing asks, "Will any microbe grow from this?" Endotoxin testing asks, "Are there bacterial toxins in this, even from dead bacteria?" The first hunts for living contamination. The second hunts for chemical residue that a live-culture test can walk right past.
How the sterility test works
The sterility test grows a sample and watches for life. Under USP General Chapter <71> (a United States Pharmacopeia standard), a portion of the batch is placed in growth media and incubated. USP <797> requires sterility testing for compounded sterile preparations given longer beyond-use dates (USP <797>, 2023). It usually runs up to 14 days. If nothing grows, the batch passes. That is a strong signal, but not the whole story.
How the endotoxin test works
The endotoxin test looks for pyrogens (in plain English: molecules that can trigger a fever). When bacteria die, pieces of their outer wall remain, and those fragments can cause reactions if injected. The bacterial endotoxin test under USP <85> uses an LAL assay (a reagent made from horseshoe crab blood) that reacts to trace endotoxin (USP <85>, 2020). It is like a smoke detector that still alarms after the fire is out.
Who this matters for
This matters for anyone receiving a compounded injectable. A certificate of analysis (a lab document summarizing test results) that lists both a sterility pass and an endotoxin pass tells a fuller safety story than either alone.
The Halftime POV
We think "sterile" should mean both tests, not one. A pharmacy that runs sterility and endotoxin testing is doing the fuller job. When we evaluate partners, we look for both results on the paperwork, because injectables leave no room for a half-answer.
Related reading:
- Sterility Testing in Compounded Peptides
- USP 795 vs USP 797 Standards
- How to Read a Certificate of Analysis
- What Happens Inside a Compounding Pharmacy
FAQ
What is the difference between sterility and endotoxin testing? Sterility testing checks whether living microbes grow from a sample. Endotoxin testing checks for fever-causing fragments left behind by bacteria, even dead ones. A product can pass one and fail the other, so quality pharmacies run both on injectables.
How are compounded peptides tested for sterility? Compounded sterile injectables are tested against USP <71>, which incubates a sample in growth media for up to 14 days. A pass means nothing grew; it does not, by itself, confirm freedom from bacterial toxins.
What is endotoxin testing for compounded drugs? Endotoxin testing looks for pyrogens from bacterial cell walls. The bacterial endotoxin test under USP <85> uses an LAL assay that reacts when trace endotoxin is present. It catches contamination a sterility test can miss.
Why do both tests matter? They catch different failures. Sterility misses toxins from bacteria already dead; endotoxin testing misses live fungi or slow growers. Running both gives a fuller safety picture for anything injected.
Disclaimer
This article is educational and is not medical advice. Compounded medications are not FDA-approved. Clinical outcomes depend on individual factors and require physician evaluation. Results vary. Halftime Health is launching soon — join the waitlist to get updates.
Get updates
Sources
- FDA, Human Drug Compounding Q&A: https://www.fda.gov/drugs/human-drug-compounding/compounding-and-fda-questions-and-answers
- FDA, Pharmaceutical Quality Resources: https://www.fda.gov/drugs/pharmaceutical-quality-resources
- United States Pharmacopeia, Compounding Standards (USP <71>, <85>, <797>): https://www.usp.org/compounding
Frequently asked questions
What is the difference between sterility and endotoxin testing?
Sterility testing checks whether living microbes can grow from a sample. Endotoxin testing checks for fever-causing fragments left behind by bacteria, even dead ones. A product can pass one and fail the other, which is why quality pharmacies run both on injectables.
How are compounded peptides tested for sterility?
Compounded sterile injectables are tested against USP General Chapter <71>, which incubates a sample in growth media to see if any microbes multiply. It typically takes up to 14 days. A pass means nothing grew; it does not, by itself, confirm the product is free of bacterial toxins.
What is endotoxin testing for compounded drugs?
Endotoxin testing looks for pyrogens (fever-causing molecules) from bacterial cell walls. The standard bacterial endotoxin test, under USP <85>, uses an LAL assay that clots or changes color when trace endotoxin is present. It catches contamination that a sterility test can miss.
Why do both tests matter?
Both matter because they catch different failures. Sterility misses toxins from bacteria that are already dead; endotoxin testing misses live fungi or slow growers. Running both gives a fuller safety picture for anything injected.
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