Identity, purity and endotoxin results can land in your inbox within a few days. Then there’s sterility, which takes two full weeks. It’s a fair question why.
The short answer is that we’re waiting for something to grow, and microbes don’t run on our schedule.
How a USP <71> sterility test works
A sterility test doesn’t look for contamination directly. Under the pharmacopeial method (USP <71>, harmonized with Ph. Eur. 2.6.1), the sample goes into two different growth media. Fluid thioglycollate medium is incubated at 30–35 °C to catch anaerobic and aerobic bacteria. Soybean-casein digest medium is kept at 20–25 °C for aerobic bacteria, yeasts and molds. Then they’re incubated for no less than 14 days and checked for any sign of growth.
Why the incubation lasts 14 days
Why so long? Many of the organisms that matter aren’t fast growers. Some species are slow by nature. Others are stressed or injured by drying, preservatives or the product itself, and they can sit quietly for days before they start multiplying. If you read the test on day five, you could easily call a sample clean that would have turned cloudy on day eleven. The 14-day window exists to give those slow organisms a chance to show up.
So cutting it short isn’t a shortcut. It just makes the test less sensitive.
Can sterility testing be done faster?
Faster methods do exist. Rapid microbiological methods that detect microbial metabolism or ATP are recognized in the pharmacopeias. Before they can replace the compendial test, though, they have to be validated as at least equivalent for the specific product. Until then, two weeks is the standard.
Planning a sterility test
If sterility matters for your research, plan for the wait. TestMyResearch offers compendial sterility testing to USP <71>. It needs its own separate vial, so include one extra when you send in your samples.
Sterility answers one question: whether anything in the vial is alive. Identity, purity and endotoxin answer others, and why testing should be the first step walks through what each one tells you before a peptide is used.
This article is for informational and research purposes only and is not medical advice.