There are two methods available for sampling surfaces: the contact plate and the swab. Only the contact plate is referenced in the E.U. requirements and in USP Chapter <1116> limits. Swabs are not thought, generally speaking, to produce quantitative data.
In the contact plate, agar contained in a specially designed Petri dish, the Rodac plate, is pressed against the surface being sampled. Microorganisms are transferred from the surface to the agar and colonies develop on incubation. Devices are available to ensure that Rodac plates are applied with a controlled, consistent, even pressure.
In the pharmaceutical industry, practically every surface in clean rooms is sampled by Rodac plates at some point. However, these plates are really only suited to sampling flat surfaces. Paradoxically flat surfaces are the easiest to clean and disinfect, and are also the least likely locations for persistent microbial colonization.
Rodac plates should not be used in critical areas such as machine surfaces around point-of-fill, on hoppers, etc., while manufacture is in progress. The risks of contamination to the product from the test procedure are greater than the benefit afforded by the data. They are usually routinely tested at the end of a filling batch or campaign.
Consideration must be given to cleaning up any residual nutrients that may have been transferred from the agar to the sampled surface. The difficulty in doing this is often exaggerated by opponents of the use of the Rodac plate. Neutralizers may have to be incorporated in the agar if it is used on disinfected surfaces.
Swabbing is done by scrubbing a moistened cotton, nylon or alginate bud across a nominal surface area. The bud is then either rolled across the surface of an agar plate, or agitated in a known volume of sterile water or other noninimical but nongrowth-supporting milieu, from which a sample is taken and plated on agar. Alternatively, the swab may be broken off into a tube of "enrichment medium," which is then incubated, and any growth streaked on agar.
As with contact plates swabs share the problems of invasiveness (the potential to contaminate a previously uncontaminated surface and thence to contaminate the product), and of the impairment of growth support of their media by disinfectant traces. Conversely, they are not restricted to flat surfaces, but are ideal for examining crevices, niches and concealed and roughened surfaces — the most difficult to clean and disinfect.
In the quest for quantitative data it is not unusual for swabs to be used in conjunction with templates defining a particular surface area to be swabbed. Regrettably this practice detracts from the best use of swabs, in the areas where templates and Rodac plates cannot be used. Swabs generate qualitative data, semiquantitative at best, nonetheless their value cannot be underestimated.
Surface sampling has its greatest value on commissioning a new clean room, or restarting an existing clean room after a shutdown period, where it has been allowed to "go nonsterile." After an initial cleanup, surfaces should be sampled; the clean room should then be disinfected and sampled again. The clean room should be thoroughly disinfected (second disinfection) and sampled a third time, and quarantined until the results from the first and second samplings are available.
If these data are satisfactory, the clean room may be released to production. If not, the room should be disinfected (third disinfection) and sampled (fourth sampling) again, but may be released to production if the data from the third sampling are satisfactory.
In routine use, surface sampling in critical areas should be done at the end of batches or campaigns. Surface samples in less critical areas may be done while sterile manufacture is in progress.
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