Sterility Testing
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Direct answer
The sterility test is a pharmacopoeial check on a sample of finished injectable and ophthalmic batches that no viable microorganism can be recovered after incubation in prescribed media, performed either by membrane filtration (product filtered, membrane rinsed to remove preservative, then transferred to media) or direct inoculation of the specimen into the media themselves. Two media cover the spectrum: fluid thioglycollate medium at 30 to 35 °C for anaerobic and facultative bacteria, and soybean-casein digest medium at 20 to 25 °C for fungi and aerobes, with incubation carried to 14 days for direct inoculation. Its central statistical truth is that sterility cannot be proven — absence of growth shows only that contamination, if present, was below the test's detection probability — which is why sterility is designed into a product through validation and sterility assurance levels rather than assured by this end-point test.
What you must remember
- Two methods: membrane filtration, preferred whenever the product filters (antimicrobial activity washes off the membrane), and direct inoculation for non-filterable, viscous or oily products.
- Media pair: fluid thioglycollate at 30 to 35 °C for bacteria including anaerobes; soybean-casein digest at 20 to 25 °C for fungi — 14 days' incubation by the direct method.
- Sampling rule (harmonised IP/BP/USP): up to 100 containers, 10 per cent or 4 whichever greater; 100 to 500 containers, 10; above 500, 2 per cent up to 20.
- Controls: each medium must pass growth-promotion tests with reference strains, and the diluent must remain sterile — a failed control invalidates the run.
- Neutralisation: preservatives and antimicrobial actives must be removed or inactivated (membrane rinsing, penicillinase-type inactivators) or a false pass results.
- Sterility assurance level: terminal sterilisation is validated to about 10⁻⁶ probability of a microbial survivor — a design target the finished test cannot demonstrate.
- On growth: the batch fails unless investigation proves laboratory error; the organism is identified, and retesting is tightly restricted.
Working through a membrane filtration run
Take a multiple-dose vial of an antibiotic injection. Pool the sampled vials, filter through two sterile 0.45 micrometre membranes, rinse each membrane three times with sterile diluent — the rinses are the whole point, since residual antibiotic would suppress growth and manufacture a false pass. Transfer one membrane into thioglycollate medium, the other into casein digest medium, and incubate at their respective temperatures, reading to the fourteenth day: clear media pass; turbidity triggers subculture, Gram stain and identification. Alongside run the controls — each medium seeded with its reference organism must show brisk growth, and the diluent must stay clear. The batch record then chains back further than the test itself: bioburden of the bulk, steriliser cycle charts, integrity testing of the sterilising filter by bubble point, and media-fill history of the line. A passed sterility test certifies only that the sample showed no recoverable organisms; the assurance of sterility was built at every earlier step, and the test merely refuses to contradict it.
The lesson examiners extract
The point papers return to is the humility of the test: a sample of a few containers cannot certify thousands, so sterility testing is a weak final gate after strong in-process controls — the inversion of what students assume. The second favourite is method selection: membrane filtration is preferred not because it is modern but because antimicrobial activity can be washed off the membrane before incubation; direct inoculation survives for unfilterable products, and its bias toward inhibition must be countered with validated inactivators. Examinees also swap the media's roles, so anchor them by organism — thioglycollate for anaerobes at blood-temperature-like conditions, casein digest for moulds and yeasts at room-temperature-like conditions. Finally, know that the test's own conditions (14-day incubation, growth promotion, neutraliser validation) are GMP-inspectable items under Schedule M documentation: a sterility test that failed its controls has tested nothing.
Frequently asked questions
Which two media are used and what does each grow?
Fluid thioglycollate medium at 30 to 35 °C recovers bacteria including anaerobes, while soybean-casein digest medium at 20 to 25 °C recovers fungi and aerobic organisms.
Why is membrane filtration the preferred method?
Because the membrane can be rinsed after filtration, removing preservatives and antimicrobial residue that would otherwise suppress growth in the medium and produce a falsely sterile result.
Can a sterility test prove that a batch is sterile?
No; it examines a small sample, so it demonstrates absence of recoverable organisms in that sample — assurance comes from validated sterilisation delivering a sterility assurance level of about 10⁻⁶.
How many containers are sampled from a large batch?
Under the harmonised rule, 2 per cent of containers up to a maximum of 20 when the batch exceeds 500 units, with proportionate numbers for smaller batches.
What happens if turbidity appears during incubation?
The medium is subcultured and the organism identified; the batch fails unless a documented laboratory error is proven, and even then retesting is restricted to defined conditions.