The probability of bacterial contamination within a given general area of a particular production facility may form the basis of QC management for that facility. This “zone theory” concept simply divides the plant (physically or theoretically) into common areas based on similar probabilities of contaminated product or equipment. Measures are typically taken to restrict movement of materials and personnel from zones of higher contamination probabilities to “clearer” zones since “dirty” materials could increase the probability of cross contamination. Each zone may be further subdivided into “sub-zones” as needed to sensibly manage the QC of that area.
Individual swipe samples, taken from a single 4-16 sq in. surface area, would typically be considered a “spot” sample within a specified zone. Multiple spot samples, taken from a pre-determined specified large area (zone), and which are later combined or pooled by the lab would qualify as “zoned” samples. This procedure is followed in some facilities in order to more cost effectively “screen” zones within the facility. Management knows that by sampling larger surface areas within a zone, the probability of discovering contamination, if present, increases proportionally. Management would consider a positive pooled sample for the microbe(s) in question to be a good indicator that the entire zone or sub-zone was contaminated. That entire area (zone or sub-zone) would then be cleaned and sanitized as needed to correct the problem. This procedure is often used to reduce laboratory costs but also because managers know that any contaminated equipment, surface or material within a sub-zone, for example, will eventually spread to other areas within that zone or possibly to other locations throughout the facility. Further, he knows that all equipment, tools or work surfaces will be cleaned whether the positive came back from an individual sample or a pooled sample.
