Before we dig into the best sample size to be taken, we must understand what type of testing will be conducted, qualitative or quantitative.
When sampling for pathogens in the manufacturing environment we typically want to know if the target organism is present or not. This type of sample is called a qualitative sample where the result is given as “present/absent”, “presumptive positive/negative” or other equivalent terms. When collecting these samples in your facility think of it as a game of seek and destroy. Sample sizes can vary each time to ensure the collection of suspicious conditions or locations such as excess moisture or an accumulation of debris/product. The thing to remember is to not overwhelm the neutralizing broth on the sampling device by taking an excessively large sample. The final objective with qualitative sampling is to provide data that your sanitation program is successful and ensure the target organisms are present or near product in your facility.
So, does the size of your sample really matter then? The answer changes to a resounding yes when are collecting a sample for a quantitative result.
A quantitative sample is often taken when conducting indicator testing such as Aerobic Plate Counts (APC), Coliform Counts, Enterobacteriaceae (EB), etc. These are bacteria or groups of bacteria that may not necessarily be hazardous to the consumer but, commonly grow in the same conditions as pathogens. This again can provide an indication of the effectiveness of your sanitation program and provide a leading indicator if there is a potential hazard in your production environment. The results for quantitative tests are given as a numeric value for the number of colonies of bacteria that are formed from your sample. You will often see the unit of Colony Forming Units (CFU). Because we are comparing numbers between samples, the size of the sample collected is vitally important and must remain constant.
Specifications are set for an acceptable/unacceptable result during baseline testing and industry knowledge. If during the baseline testing a 4 inch by 4 inch sample is always taken you are collecting 16 square inches and getting a result to set your specification. If you proceed to collect an 8 inch by 8 inch sample during your routine environmental monitoring, you are now collecting 64 square inches. This sample is 4 times larger than the samples used to set the specification. There may be the same number of bacteria during both samples but, we could then expect a result 4 times higher in the routine sample and an out of specification (OOS) result.
The moral of the story, when collecting a sample for a quantitative result always collect a consistent sample size.