Sterility testing (revision 2)

Sterility testing determines whether a sample yields microbial growth when incubated in defined media under defined conditions. A passing result does not establish that a batch is sterile; it establishes that the units tested did not yield growth, which is a statistical statement about the batch rather than a property of it.^[1]

This limitation is intrinsic and well recognised. Testing a small number of units from a large batch has limited power to detect low-level contamination, and for this reason sterility assurance in manufacture rests on process validation and environmental control rather than on end-product testing.^[2]

Two methods are recognised. Membrane filtration passes the sample through a 0.45 μm filter that retains organisms, washes away any inhibitory substance, and incubates the membrane in growth media; it is preferred where the sample can be filtered. Direct inoculation adds the sample to media directly and is used where filtration is impractical.^[1]

Two media are used in parallel: a soybean–casein digest medium incubated at 20–25 °C for fungi and aerobic organisms, and a fluid thioglycollate medium at 30–35 °C for anaerobes and aerobes. Incubation is for not less than fourteen days, which is why a sterility result is never available quickly.